The Latics, who have already wrapped up automatic promotion, came from behind to earn a point with Michael Jacobs’ second-half goal cancelling out Joe Pigott’s opener for the Dons.
That leaves former Pompey boss Paul Cook’s men two points ahead of Blackburn with just one game to play after Rovers were beaten 1-0 at Charlton.
Tony Mowbray’s men have also sealed an instant return to the Championship.
But they were downed by Jason Pearce’s first-half goal for the Addicks, who moved closer to sealing a play-off place.
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Managerless MK Dons will be playing in League Two for the first time in a decade next season after their relegation was confirmed with a 2-0 loss at home to Scunthorpe.
They came into the game needing an unlikely set of results to pull off an escape, but first-half goals from Rory McCardle and Ivan Toney sent them down.
The win for Scunthorpe means their midweek clash with Plymouth will be vital in the chase for the play-offs after Argyle’s home win over Rotherham.
The Millers are guaranteed fourth spot and they led through Richie Towell, only for Joel Grant and Graham Carey to win it after the break.
Plymouth manager Derek Adams praised the spirit of his team as they remained in the hunt.
He told the Plymouth Herald: ‘We have got tremendous character throughout the side.
‘We have had a lot of setbacks this season that they have had to deal with, and these players deal with it all the time.
‘This is a prime example. We had to win the game and we get the three points.
‘I thought we started the game very well, on the front foot, and kept a lot of possession.
‘We were probably unfortunate not to have gone ahead in the game.
‘We made a mistake when Zak Vyner was too casual on the ball and it allows them to get in and Richie Towell took the goal very well.
‘We knew at half-time that if we kept on probing we would get chances in the game, and that’s what happened.
‘We kept plugging at them and eventually we scored a very good goal. A flick on from Graham (Carey) and a great finish from Joel Grant.
‘That’s two in two for him, so it’s great for him as the centre-forward.
‘In the end, we get the penalty kick and Graham puts it in the top corner.’
Walsall claimed a vital 1-0 win in last-gasp fashion which puts them in touching distance of safety and effectively relegates Northampton.
George Dobson bagged an injury-time winner to move the Saddlers three points clear of the drop zone with two games left to play.
The Cobblers are three points adrift of safety with a vastly inferior goal difference and only one game remaining.
That win sent Rochdale back into the bottom four after they were beaten 2-1 at Oxford, despite having led through Brad Inman’s opener.
Oldham are a point clear of Dale after a 0-0 draw with Doncaster and their fate will be decided on the final day where they play Northampton, who will need to win by nine goals.
Shrewsbury, assured of third place, drew 1-1 with Blackpool.
Peterborough enjoyed a 2-0 success over Fleetwood, with Jack Marriott scoring his 27th league goal of the season, while two late goals saw Bristol Rovers and Gillingham draw 1-1.