Posh saw plenty of the ball, but created precious little against dogged and determined opponents who have hauled themselves out of the relegation zone in recent weeks.
But Posh were comfortable defensively until conceding twice in three minutes early in the second-half. Goalkeeper Christy Pym who has enjoyed a fine season, was culpable for both of them.
Posh huffed and puffed in reply. They created two chances for substitute Mo Eisa, but he missed them both before another replacement Harrison Burrows bagged his first senior goal in the final seconds. It was too little too late.
There was a boost for Posh before the game as top scorer Jonson Clarke-Harris was declared to continue his ever-present status in League One matches in an unchanged starting line-up.
Burton made two changes to their starting line up, but retained former Posh star Michael Bostwick as a centre-back.
Posh looked slick in the opening stages, once Pym had raced out of his goal to clear from Kane Hemmings after 10 seconds. In the second minute a couple of fine passes sent Sammie Szmodics into the Burton area, but his low cross was well cut out at the expense of a corner, one of many Posh won in the opening quarter.
From one of them Ethan Hamilton saw a shot from 12 yards well blocked and Szmodics volleyed wide from another from the edge of the area.
The clearest Posh chance arrived in the 15th minute when an innocuous shot from Szmodics was spilled by home ‘keeper Ben Garrett. Clarke-Harris pounced, but as he prepared to pull the trigger he slipped and the chance vanished.
Burton’s attacking plans appeared to centre around long throws and set-pieces, all of which were defended well, but Pym came to the Posh rescue in the 28th minute after a bizarre passage of play.
Burton passed neatly to get Danny Rowe on the ball and as he advanced on the Posh area both Frankie Kent and Mark Beevers slipped. Rowe was able to pick his spot, but Pym read it superbly to make a terrific save.
The longer the game went on, the scrappier it became which suited a Burton side who are fighting hard for League One survival. They were aggressive and pressed well, although Posh passed round them enough in midfield areas to create more than they actually managed.
Posh started the second-half slowly and Beevers had to react well to block a Lucas Akins 20-yarder. When Posh did break out Clarke-Harris almost capitalised on hesitation between Bostwick and Garrett.
But then it all went wrong as Burton scored twice in three minutes and Pym, who has enjoyed an excellent season, won’t be happy with either of them.
In the 55th minute Jonny Smith was given too much time to cross from inside the area, but it should have been an easy collection for Pym. Instead he spilled it and Kane Hemmings tapped home from close range to celebrate his third goal this season against Posh.
A free kick given away by Beevers then enabled the home side to double their lead. Tom Hamer won the cross at the back post and Hayden Carter volleyed home as Pym and Clarke-Harris got in each other’s way.
Posh responded with plenty of possession, but couldn’t find a telling pass or cross. Most centres were met at the near post by Bostwick who took great pleasure in booting the ball out of play.
Posh withdrew Clarke-Harris, a fitness doubt before the game, and left-back Dan Butler, and changed shape to wing-backs.
Burton allowed Posh the ball, but competed whenever danger threatened and they normally came away with the ball.
Eisa had his first opportunity to score 15 minutes from time after a neat little pass from Dembele, but although he beat Garratt, his shot lacked the pace to get past Carter who cleared off the line easily.
And 10 minutes from time Eisa was sent through by a long Kent through ball, but lobbed the ball over the bar as well as the goalkeeper.
Hamilton fired in a strong free kick which was saved in the 94th minute before substitute Burrows fired home with virtually the last kick of the game.
With that Posh accepted a first defeat since the end of January with a game against new leaders Hull City to come.
Posh: Christy Pym, Dan Butler (sub Harrison Burrows, 68 mins), Mark Beevers, Nathan Thompson, Frankie Kent, Joe Ward, Reece Brown, Ethan Hamilton, Siriki Dembele, Sammie Szmodics, Jonson Clarke-Harris (sub Mo Eisa, 64 mins).
Unused subs: Dan Gyollai, Ricky-Jade Jones, Niall Mason, Louis Reed, Idris Kanu.
Burton: Ben Garratt, John Brayford, Hayden Carter, Michael Bostwick, Tom Hamer, Jonny Smith (sub Owen Gallacher, 90 + 3 mins), Ryan Edwards, Michael Mancienne, Kane Hemmings (sub Josh Parker, 76 mins), Lucas Akins (sub Terry Taylor, 71 mins), Danny Rowe (sub Joe Powell, 76 mins).
Unused subs: Kieran O’Hara, Josh Earl, Mike Fondop-Talum.
Goals: Burton - Hemmings (55 mins), Carter (57 mins).
Cautions: Posh - Beevers (foul).
Burton - Smith (foul), Brayford (foul), Carter (delaying the restart).
Referee: Carl Brook 7.