MICHAL Helik lashed home a powerful volley to bring Barnsley a fifth straight Championship win.
The Polish defender's fifth goal of the season a quarter of an hour into the second half made the difference after the sides swapped strikes in the opening six minutes of the match.
First top scorer Cauley Woodrow put the home team ahead with his 12th of the season, then Mason Bennett levelled as Millwall counter-attacked.
The Lions continued to look lively, but overall, Valerien Ismael's men deserved the three-point haul which took them to 51 for the season, past the head coach's stated target to ensure there isn't a repeat of last season's survival scrap.
However, rather than looking over their shoulders, the Oakwell side are closer to the play-offs after extending their unbeaten league run to seven matches.
Barnsley made two changes to the starting side following Wednesday's 2-0 home win over Stoke City.
Michael Sollbauer came in for Toby Sibbick at the back and Daryl Dike for Dominik Frieser up front as the Reds went with their usual 3-4-3 formation.
Millwall lined up 5-3-2, bringing in Shaun Williams and Mason Bennett for former Barnsley striker Tom Bradshaw and Iceland international Jon Dadi Bodvarsson after Tuesday's 1-1 draw at Luton Town.
The game got off to an action-packed start with two goals in the opening six minutes.
First Woodrow put the Reds ahead from close range after Mads Andersen touched on an Alex Mowatt free-kick, with referee Leigh Doughty rejecting Millwall's offside claims.
But the lead lasted only four minutes, as after another Barnsley free-kick, the visitors cleared and broke upfield, with Jed Wallace releasing Bennett, who advanced and beat Brad Collins with a left-foot shot.
Reds wing-back Callum Brittain almost caught Millwall out with an inswinging corner, but Bartosz Bialkowski managed to punch the ball clear.
Barnsley were doing most of the attacking, with Mowatt having a shot blocked and Carlton Morris glancing a header across the face of goal, although Millwall continued to look dangerous on the counter-attack, and Bennett had a header blocked.
Callum Styles, who shot home from distance during the Reds' win over Stoke, tried a similar effort on 39 minutes, but this time was wide of the target.
Barnsley made two charges at the break, Sibbick for Sollbauer and Aapo Halme for Romal Palmer.
Both goalkeepers were called on early in the second half, Collins to thwart Bennett, who glanced a header goalwards from Williams' cross, and Bialkowski to tip Mowatt's well-struck 20-yard free-kick over the bar.
The Reds were back in front on the hour, when Helik volleyed home after Millwall failed to deal with Mowatt's awkward high delivery and Andersen headed the ball on.
Halme drilled a shot too high as Barnsley pushed forward in a bid to extend their advantage, although the home side breathed a sigh of relief when Scott Malone's cross whistled across the face of goal and was deflected for a corner.
Morris then came close with a header from Mowatt's cross, but Bialkowski made a good save.
Barnsley: Collins, Sollbauer (Sibbick, HT), Helik, Andersen, Brittain, Palmer (Halme, HT), Mowatt, Styles (J Williams, 81), Dike (Adeboyejo, 75), Woodrow (Frieser, 90), Morris. Unused: Kane, Chaplin, Oduor, Walton.
Millwall: Bialkowski, Romeo, Hutchinson, Pearce (Bodvarsson, 85), Evans, Malone, Woods, Williams (Ferguson, 85), Thompson (Smith, 75), Bennett (Burey, 85), Wallace (Bradshaw, 90). Unused: McNamara, Tiensia, Mitchell, Fielding.
Referee: Leigh Doughty (Fylde).
AS IT HAPPENED
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2:00pm
Welcome to our coverage of Barnsley's home match in the Championship against Millwall. The Reds are hoping for their fifth successive league win, while the visitors have gone unbeaten in nine.
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2:04pm
Barnsley's side features two changes from the side that beat Stoke City on Wednesday - Michael Sollbauer comes in for Toby Sibbick, Daryl Dike for Dominik Frieser. Collins; Sollbauer, Helik, Andersen; Styles, Mowatt, Palmer, Brittain; Dike, Woodrow, Morris. Subs: Walton, J Williams, Sibbick, Halme, Oduor, Kane, Chaplin, Frieser, Adeboyejo.
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2:08pm
Millwall: Bialkowski; Hutchinson, Williams, Pearce; Malone, Thompson, Woods, Romeo; Bennett, Wallace. Subs: Fielding, Bradshaw, Smith, Ferguson, Bodvarsson, McNamara, Mitchell, Tiensia, Burey.
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2:09pm
The visitors also make two changes from their midweek draw with Luton Town - Mason Bennett and George Evans come in for Jon Dadi Bodvarsson and ex-Red Tom Bradshaw.
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2:13pm
The Reds, unbeaten in six, are now one point off the play-off spots after their 2-0 success over Stoke City thanks to goals from Callum Styles and Daryl Dike (his first in a Barnsley shirt). They have a game in hand over seventh-placed Cardiff City and sixth-placed Reading - and two over Bournemouth who are fifth.
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2:15pm
This is their sixth game in a busy February that started with a 1-0 defeat at home to Premier League opponents Chelsea in the FA Cup fifth round. Since then, Valerien Ismael's side haven't lost and have kept three clean sheets, becoming one of the form sides in the league.
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2:19pm
“It will be a fight with intense duels,” said Ismael ahead of today's match. “We will need to show a big mentality to enjoy the fight – it will be important to be clinical because we will have our chance to score and we have to do it.
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2:23pm
“At the same time, on set-pieces and on the ball in our box, we have to be clinical in our defence because Millwall are very strong in transition and set-pieces. We have to be very good in this area.”
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2:55pm
Five minutes to kick-off on a surprisingly pleasant Saturday afternoon.
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3:02pm
1 Kick-off from the Reds and immediately a free kick is won and sent upfield by Ryan Woods. It goes out of play for a goal kick.
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3:03pm
2 Cauley Woodrow taps in from an Alex Mowatt free kick!
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3:06pm
2 Shouts for offside from the Millwall defenders, but the linesman keeps his flag down. Alex Mowatt's free kick from the centre of the pitch wasn't dealt with and Cauley Woodrow could tap it in under Bartosz Bialkowski.
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3:07pm
6 Mason Bennett nets to Brad Collins' left after advancing with the ball.
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3:09pm
6 The striker had the run on Barnsley's defence after a free kick up the other end. The Reds were punished for leaving men upfield as Millwall countered well. Good run from Mason Bennett and well finished across Brad Collins' goal.
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3:12pm
11 Frantic start at Oakwell. The visitors are putting together some good moves through the midfield while the Reds - playing into the sun - play their typical long ball game.
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3:14pm
13 Scott Malone and Jed Wallace combine with a one-two on the left which is just about dealt with, before Carlton Morris receives the ball on the counter and takes on Alex Pearce on the left wing, winning a corner. Barnsley do little with the set piece.
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3:20pm
19 Jed Wallace wins a free kick on the left, right by the corner flag, after a throw-in from the visitors. Michael Sollbauer with the foul. The resulting free kick is eventually caught by Brad Collins after being sent back into the box.
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3:26pm
25 After two goals in the first ten minutes, it's become a pretty scrappy affair with no clear chances for either side.
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3:29pm
27 Some stern words from referee Leigh Doughty to Reds head coach Valerian Ismael after a seemingly late whistle following a foul on Millwall's Scott Malone right by the dug-outs. Ismael was visibly angry about the timing of the call.
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3:32pm
31 Callum Styles wins a free kick with some smart footwork on the left but it's cleared and goes back to Brad Collins.
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3:34pm
33 Alex Mowatt's somewhat mis-hit effort on the half volley is tipped wide by Bartosz Bialkowski. The skipper got the chance just on the edge of the box, right in the middle of goal, after a few loose headers.
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3:36pm
35 Carlton Morris rises highest to an Alex Mowatt corner kick from the right, but it just glances past his head and goes out wide of the far post.
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3:38pm
37 Mahlon Romeo does well to keep the ball in play, avoiding several challenges, but no one can get to his cross and it's caught by Brad Collins.
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3:39pm
38 Carlton Morris takes a crack and it's deflected on to the roof of the net for a Reds corner, but the referee blows his whistle for a foul on the Millwall defender.
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3:41pm
40 Callum Styles almost replicates his strike on Wednesday night against Stoke City. He receives the ball from Alex Mowatt in midfield and takes a touch before firing it wide of the right hand post from the inside left channel.
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3:43pm
42 Mads Andersen nods the ball towards goal from a free kick from well out, but it lacks power and is caught by Bartosz Bialkowski.
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3:46pm
45 Michael Sollbauer gives away a foul on Jed Wallace on the visitors' left with a late challenge for a loose ball. It's easily cleared when it comes into the box.
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3:46pm
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3:47pm
HALF-TIME: Barnsley 1, Millwall 1. A lively start, but little action since those opening two goals.
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4:04pm
Half-time subs - Romal Palmer and Michael Sollbauer off.
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4:04pm
Aapo Halme and Toby Sibbick.
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4:04pm
46 We're away for the second half.
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4:06pm
47 An early free kick as Aapo Halme is caught just past the halfway line as Barnsley try to counter. It's nodded on by Halme after Alex Mowatt's delivery and Michal Helik stretches to get a foot to the knock-on, but can't get there.
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4:10pm
50 Some good work by Shaun Williams on the left for Millwall as the midfielder beats his man and crosses it in, and it's headed on acrobatically by Mason Bennett. His effort is well saved by Brad Collins.
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4:12pm
53 Alex Mowatt goes close from a free kick that's tipped over the bar by Bartosz Bialkowski. Callum Styles won the free kick with some neat footwork.
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4:14pm
55 A nice idea from a free kick on the right, Alex Mowatt playing it along the ground for Callum Brittain to chase on the right post. But the wing-back can't quite get to the ball.
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4:16pm
57 Some sustained pressure sees Cauley Woodrow nick the ball from under Shaun Hutchinson's feet and play a one-two with Carlton Morris before firing the ball into the box. It's not dealt with and the Reds win a few more headers, but Aapo Halme eventually sends the ball out for a goal kick.
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4:18pm
59 Michal Helik nets for the Reds!
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4:21pm
59 Michal Helik, who remained up following an earlier set piece, fires in from close range with a volley. He was played in by centre-back partner Mads Andersen with a neat header.
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4:25pm
65 The Reds are playing a lot better than they did following the early goal in the first half, with more composed passing.
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4:28pm
68 A good move from Barnsley starts with Callum Styles and Carlton Morris who plays it across to the right for Callum Brittain, who takes a touch and crosses in. It's cleared as far as Aapo Halme, whose strike is a powerful one but sails well over the bar.
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4:31pm
71 Scott Malone fires across goal from the left side of the box after a free kick. Not clear if it's a cross or a shot, but Brad Collins has to be alert to parry it wide for a corner. The Reds clear.
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4:33pm
73 Mason Bennett outmuscles Michal Helik on the left of goal and holds him off long enough to strike towards goal. His side-footed effort goes over the bar as he leans back a little too much.
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4:33pm
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4:33pm
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4:34pm
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4:34pm
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4:39pm
80 Alex Mowatt swings a free kick in and Carlton Morris heads it towards the bottom left corner, but it's well saved one-handed by Bartosz Bialkowski.
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4:40pm
80 Carlton Morris had won the free kick with some dribbling on the left wing, the Reds stringing together a few passes in that area.
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4:40pm
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4:40pm
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4:43pm
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4:43pm
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4:43pm
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4:44pm
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4:44pm
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4:44pm
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4:49pm
89 Cauley Woodrow booked for kicking the ball into the stands after it went out of play. The forward very nearly played Victor Adeboyejo in after the substitute gave it to Woodrow on the left.
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4:49pm
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4:50pm
90+1 Jed Wallace fires wide on the turn from the right of goal, but the Millwall forward was called offside anyway.
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4:50pm
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4:50pm
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4:51pm
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4:51pm
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4:52pm
90+3 Victor Adeboyejo takes on two defenders on the left, right by the touchline, and collides with a third in the box - but the referee doesn't call the foul.
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4:53pm
90+5 The added four minutes up and Barnsley with a corner on the right. They keep it there.
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4:54pm
FULL-TIME Barnsley 2, Millwall 1. A fifth straight win for the Reds, who worked hard for it.
