Grant McCann struck a late winner to dent Leicester's hopes of an automatic promotion place and give bottom side Peterborough a priceless win.
The Foxes took the lead when winger Ben Marshall fired into the top corner from 10 yards.
Posh levelled with 16 minutes remaining thanks to a superb 20-yard volley by on-loan Manchester United player Davide Petrucci.
McCann then drilled in late on to the delight of the home support.
The result leaves Darren Ferguson's side only four points from safety with a game in hand on the seven teams above them.
At the other end of the table, the Foxes remain on 53 points with Watford, who drew with Crystal Palace on Friday, and Hull, who lost at Brighton in Saturday's late game.
Nigel Pearson's side also missed out on the chance to record a sixth successive win for the first time in two decades.
They created plenty of other chances in the match, with Marshall's free-kick forcing a good save from Robert Olejnik, who then made a one-handed stop to prevent David Nugent from scoring.
And after Marshall gave Leicester the lead, Jamie Vardy, in the team for the injured Chris Wood, failed to double the advantage by fractions when he could not stretch far enough to convert Nugent's driven cross.
Posh also created other scoring opportunities, with Leicester captain Wes Morgan making heroic blocks to deny Lee Tomlin and Petrucci, while Kasper Schmeichel saved McCann's free-kick and blocked George Boyd's strike.
Peterborough boss Darren Ferguson: "It has been a very good day to get that performance and result.
"We were the better team from the off and scored a couple of outstanding goals to get the win.
"We've always had the belief in the dressing room that we can stay up and hopefully a victory like this will give that belief to the fans.
"I can certainly go home and enjoy the rest of my birthday."
Leicester manager Nigel Pearson: "There's no doubt it goes down as a missed opportunity.
"When you get in front in a game like this, you have to close it out and we failed.
"We certainly created enough good chances to do that but I felt we didn't work hard enough to keep a clean sheet at the other end.
"That is not something you will hear me say about my players too often but I felt Peterborough had more drive than us, especially after they had got themselves level."