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."
- 01 Olejnik
- 05 Zakuani
- 19 Little
- 06 Bostwick
- 11 McCann
- 14 Rowe
- 23 Petrucci
- 44 Mendez-Laing (Pritchard - 75' )
- 08 Tomlin
- 10 Boyd
- 34 Gayle
- 13 Day
- 04 Brisley
- 17 Newell
- 18 Ferdinand
- 20 Knight-Percival
- 26 Payne
- 33 Pritchard (Ferdinand - 90' )
- 01 Schmeichel
- 02 De Laet Booked
- 03 Konchesky
- 05 Morgan
- 15 M Keane
- 04 Drinkwater
- 07 Marshall (Dyer - 78' )
- 10 King
- 24 Knockaert
- 09 Vardy (Waghorn - 70' )
- 35 Nugent
- 13 Logan
- 11 Dyer
- 16 James
- 17 Gallagher
- 19 Wellens
- 14 Waghorn
- 29 Futacs
Live Text Commentary
Full Time The referee ends the match.90:00 +4:56
Kasper Schmeichel restarts play with the free kick.90:00 +4:56 Substitution
Substitution Kane Ferdinand is brought on as a substitute for Alex Pritchard.90:00 +4:56
The official flags Dwight Gayle offside.90:00 +2:37
Inswinging corner taken left-footed by Anthony Knockaert played to the near post, clearance by Lee Tomlin.90:00 +2:27
The ball is crossed by Anthony Knockaert, clearance by Robert Olejnik.90:00 +1:56
The official flags Dwight Gayle offside. Kasper Schmeichel takes the free kick.90:00 +0:15
Lee Tomlin takes a shot.89:47
Martyn Waghorn produces a left-footed shot from inside the area that clears the crossbar.87:46
Assist on the goal came from George Boyd.87:46 Goal scored
Goal! - Grant McCann - Peterboro 2 - 1 Leicester Grant McCann finds the net with a goal from just inside the area to the bottom left corner of the goal. Peterborough 2-1 Leicester.82:59
The ball is sent over by Alex Pritchard, clearance by Andy King.82:32
Anthony Knockaert takes a shot. Save made by Robert Olejnik.81:18
The ball is sent over by Tommy Rowe, clearance by Martyn Waghorn. Inswinging corner taken left-footed by Grant McCann, Andy King manages to make a clearance.80:08
Unfair challenge on Tommy Rowe by David Nugent results in a free kick. Direct free kick taken by Robert Olejnik.79:36
Martyn Waghorn takes a shot. Save by Robert Olejnik.78:17
Grant McCann restarts play with the free kick. Shot by Michael Bostwick from a long way out goes over the crossbar.78:17 Booking
Booking Ritchie De Laet booked for unsporting behaviour.78:05
Handball by Ritchie De Laet.77:50
Lee Tomlin takes a shot. Save made by Kasper Schmeichel.
Substitution (Leicester) makes a substitution, with Lloyd Dyer coming on for Ben Marshall.76:53
Inswinging corner taken right-footed by Ben Marshall.75:08
Wes Morgan fouled by Dwight Gayle, the ref awards a free kick. Direct free kick taken by Kasper Schmeichel.74:15 Substitution
Substitution Alex Pritchard joins the action as a substitute, replacing Nathaniel Mendez-Laing.73:13 Goal scored
Goal! - Davide Petrucci - Peterboro 1 - 1 Leicester Davide Petrucci gets on the score sheet with a goal from just outside the area to the top right corner of the goal. Peterborough 1-1 Leicester.72:10
The offside flag is raised against Martyn Waghorn. Indirect free kick taken by Gabriel Zakuani.71:22
Foul by Martyn Waghorn on Grant McCann, free kick awarded. Michael Bostwick takes the direct free kick.70:26
Free kick awarded for an unfair challenge on Danny Drinkwater by Grant McCann. Direct free kick taken by Paul Konchesky.69:17
Free kick taken by Kasper Schmeichel.69:17 Substitution
Substitution Jamie Vardy leaves the field to be replaced by Martyn Waghorn.69:17
The assistant referee flags for offside against Nathaniel Mendez-Laing.68:00
George Boyd takes a shot. Save made by Kasper Schmeichel. Corner taken left-footed by Grant McCann to the near post, clearance made by Wes Morgan. Effort from deep inside the area by George Boyd misses to the left of the target. Corner taken by Grant McCann played to the near post, clearance by Michael Keane.67:04
Anthony Knockaert produces a left-footed shot from just outside the box that goes wide left of the goal.66:14
Centre by Michael Bostwick. Davide Petrucci takes a shot. Blocked by Wes Morgan. Lee Tomlin takes a shot. Ritchie De Laet gets a block in.65:36
Ritchie De Laet concedes a free kick for a foul on Tommy Rowe. Davide Petrucci takes the free kick.64:28
Lee Tomlin takes a shot. Blocked by Wes Morgan.62:57
Jamie Vardy takes the chance to get an effort at goal. Save made by Robert Olejnik. Inswinging corner taken by Ben Marshall from the right by-line, save by Robert Olejnik.61:19
Free kick awarded for a foul by Tommy Rowe on Anthony Knockaert. The free kick is swung in left-footed by Anthony Knockaert. The offside flag is raised against David Nugent. Robert Olejnik takes the free kick.60:36
Foul by Tommy Rowe on Andy King, free kick awarded. Danny Drinkwater restarts play with the free kick.59:13
Grant McCann takes the outswinging corner, Kasper Schmeichel makes a save.58:40
Nathaniel Mendez-Laing crosses the ball, Kasper Schmeichel makes a save.57:55
The ball is swung over by Ritchie De Laet.57:44
A cross is delivered by Ritchie De Laet, Michael Bostwick makes a clearance.56:58
The ball is sent over by David Nugent.53:54
The referee blows for offside against Jamie Vardy. Free kick taken by Robert Olejnik.53:23
A cross is delivered by David Nugent, clearance by Mark Little.51:27
Danny Drinkwater provided the assist for the goal.51:27 Goal scored
Goal! - Ben Marshall - Peterboro 0 - 1 Leicester Ben Marshall scores a goal from deep inside the penalty box to the top left corner of the goal. Peterborough 0-1 Leicester.51:04
Andy King sends in a cross, clearance made by Michael Bostwick.49:57
Jamie Vardy is caught offside. Robert Olejnik restarts play with the free kick.47:08
Anthony Knockaert challenges George Boyd unfairly and gives away a free kick. Grant McCann takes the direct free kick.46:37
The official flags Lee Tomlin offside. Kasper Schmeichel takes the indirect free kick.45:48
Centre by Jamie Vardy, save made by Robert Olejnik.45:12
Grant McCann gives away a free kick for an unfair challenge on Danny Drinkwater. Direct free kick taken by Paul Konchesky.45:01
The referee blows his whistle to start the second half.45:00 +1:06 Half time
Half Time The ref blows to signal half-time.42:22
Davide Petrucci gives away a free kick for an unfair challenge on Danny Drinkwater. Free kick crossed right-footed by Ben Marshall from left wing, save made by Robert Olejnik.40:40
The ball is crossed by George Boyd, Paul Konchesky manages to make a clearance.39:42
The ball is crossed by Jamie Vardy, Grant McCann makes a clearance. The ball is sent over by Anthony Knockaert, clearance by Mark Little.37:30
Lee Tomlin takes a shot. Blocked by Wes Morgan.32:42
The ball is sent over by Paul Konchesky, David Nugent takes a shot. Robert Olejnik makes a brilliant save.31:12
Lee Tomlin produces a left-footed shot from the edge of the box and misses to the right of the goal.30:05
The ball is crossed by David Nugent, Gabriel Zakuani manages to make a clearance. Andy King takes a shot. Michael Bostwick gets a block in. Danny Drinkwater takes a shot. Michael Bostwick gets a block in. Paul Konchesky takes a shot. Robert Olejnik makes a save.28:48
The referee gives a free kick against Paul Konchesky for handball. The free kick is delivered left-footed by Grant McCann from right by-line.25:40
Anthony Knockaert takes a shot. Blocked by Gabriel Zakuani.24:43
Free kick awarded for an unfair challenge on Mark Little by Danny Drinkwater. Direct free kick taken by Grant McCann.20:26
Free kick awarded for an unfair challenge on Mark Little by Ben Marshall. Grant McCann has a direct shot on goal from the free kick, save by Kasper Schmeichel. Grant McCann takes the inswinging corner, Tommy Rowe concedes a free kick for a foul on Ritchie De Laet. Kasper Schmeichel restarts play with the free kick.16:56
Anthony Knockaert takes a shot. Tommy Rowe gets a block in.15:52
Foul by Tommy Rowe on Ritchie De Laet, free kick awarded. Danny Drinkwater takes the free kick.15:07
The ball is delivered by George Boyd, save by Kasper Schmeichel.13:47
George Boyd takes a shot. Blocked by Wes Morgan. Outswinging corner taken by Grant McCann, Andy King manages to make a clearance.13:35
The ball is swung over by Dwight Gayle, Michael Keane makes a clearance.13:12
A cross is delivered by Lee Tomlin, Paul Konchesky manages to make a clearance.12:55
Tommy Rowe produces a cross, clearance made by Wes Morgan.12:29
The ball is crossed by George Boyd, clearance by Wes Morgan.11:46
Corner taken by Grant McCann, clearance made by Ritchie De Laet.10:53
Centre by Mark Little, blocked by Paul Konchesky. Inswinging corner taken left-footed by Grant McCann, Andy King makes a clearance.9:53
Michael Keane fouled by Davide Petrucci, the ref awards a free kick. Paul Konchesky takes the free kick.8:39
The ball is sent over by Paul Konchesky.6:13
Nathaniel Mendez-Laing concedes a free kick for a foul on Ben Marshall. Strike comes in from Ben Marshall from the free kick, Robert Olejnik makes a save. Inswinging corner taken by Ben Marshall from the left by-line played to the near post, Davide Petrucci manages to make a clearance.3:42
Free kick awarded for an unfair challenge on Wes Morgan by Lee Tomlin. Kasper Schmeichel restarts play with the free kick.2:13
Paul Konchesky sends in a cross, clearance by Gabriel Zakuani.2:02
Ritchie De Laet produces a cross, clearance by Gabriel Zakuani.0:27
Unfair challenge on Lee Tomlin by Danny Drinkwater results in a free kick. Free kick crossed left-footed by Grant McCann from left channel, clearance made by Danny Drinkwater.0:00
The match gets underway.
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