Manchester City's title defence suffered a huge blow on the south coast as they gifted Southampton victory with a series of glaring errors at the back.
Gareth Barry gave the ball away for Saints' first goal, with Jason Puncheon following up to slot home after Jay Rodriguez's shot was saved.
Steven Davis made it 2-0 after Joe Hart made a hash of Rickie Lambert's shot.
Edin Dzeko replied on the break but City's misery was complete when Barry inexplicably passed into his own net.
It was a disastrous defeat for Roberto Mancini's men, which means leaders Manchester United can extend their lead at the top of the Premier League to 12 points with 12 games to go if they beat Everton at Old Trafford on Sunday.
Saints, meanwhile, move six points clear of the relegation zone. They thoroughly deserved their first win under new boss Mauricio Pochettino but this is a game that will be remembered for City's defensive horror show, coupled with careless passing and lethargic attacks, which leaves their hopes of retaining their crown in tatters.
City had arrived at St Mary's with the best defensive record in the top flight but, in the absence of captain Vincent Kompany and his regular partner Matija Nastasic, they showed precious little authority in their back-four.
Javi Garcia, usually a defensive midfielder but being used at centre-back, was particularly uncertain with his positioning, and Saints punished him ruthlessly when they took the lead after seven minutes.
Manchester City's day to forget
- This was only the seventh time that Manchester City have conceded three goals (never more) in a Premier League game under Roberto Mancini.
- Man City have suffered just two defeats in the Premier League that were heavier than this under Roberto Mancini - 0-3 at home to Arsenal in October 2010 and 0-3 at Liverpool in April 2011.
Stats courtesy of Opta
Puncheon stole the ball off Barry and, when Garcia joined Joleon Lescott in closing him down, played in Rodriguez in space on the edge of the area. Hart saved his effort but Puncheon had continued his run and rolled the rebound into an unguarded net.
Southampton's industry and discipline did not allow City any opportunities to respond and things got worse for the visitors when the usually dependable Hart made a shocking error, dropping Lambert's shot and letting it squirm between his legs for Davis to turn it over the line.
Saints' tactics were simple - press high up the pitch and attack with pace when they forced mistakes. It almost brought them a third goal, when Rodriguez volleyed narrowly wide, before City finally found some rhythm before half-time.
They replied with a classic counter-attack, started by Dzeko when he headed a corner clear on the edge of his own area, and finished by the Bosnian too after he slid in to scoop home Pablo Zabaleta's cross.
That gave City hope of staging another comeback similar to the one they mounted at the Etihad on the opening weekend of the season when they trailed Saints 2-1 but won 3-2. In truth, they never came close to a repeat.
Saints could have had a penalty before the break when Yaya Toure appeared to barge Rodriguez over in the box, but referee Martin Atkinson awarded a corner instead.
That decision did not make a difference in the end as the final blow was administered by their own hand or, to be precise, Barry's left foot.
There was no danger and no-one near Barry when Lambert's cross reached him in the middle of City's area but his attempt to play the ball back to Hart resulted in him placing the ball into the bottom corner of his own net instead.
Mancini threw on James Milner, then Aleksandar Kolarov and Maicon to try to find a way back into the game, but to no avail. His side were short of ideas and energy, and were out-passed and out-played.
The closest City got to pulling another goal back saw Sergio Aguero denied by Artur Boruc's instinctive save, and the game ended with Saints looking more likely to extend their lead than surrender it.
- 31 Boruc
- 02 Clyne
- 03 Yoshida
- 05 Hooiveld
- 13 Fox (Richardson - 71' )
- 04 Schneiderlin
- 08 S Davis (Ward-Prowse - 80' )
- 18 Cork
- 42 Puncheon (Lallana - 61' )
- 07 Lambert Booked
- 09 Rodriguez
- 01 K Davis
- 15 Forren
- 22 Richardson
- 16 Ward-Prowse
- 20 Lallana
- 27 Chaplow
- 19 Lee
- 01 Hart
- 05 Zabaleta
- 06 Lescott (Kolarov - 66' )
- 22 Clichy Booked
- 08 Nasri (Milner - 55' )
- 14 Garcia
- 18 Barry
- 21 Silva (Maicon - 73' )
- 42 Y Toure Booked
- 10 Dzeko
- 16 Aguero
- 30 Pantilimon
- 03 Maicon
- 13 Kolarov
- 28 K Toure
- 07 Milner
- 11 Sinclair
- 17 Rodwell
Live Text Commentary
Full Time The referee brings the game to a close.90:00 +4:39
James Milner produces a cross, clearance by Jos Hooiveld.90:00 +1:40
Unfair challenge on Jack Cork by Gareth Barry results in a free kick. Maya Yoshida restarts play with the free kick.90:00 +0:17
James Milner delivers the ball.89:41
Inswinging corner taken by Aleksandar Kolarov, save by Artur Boruc.86:20
Centre by Sisenando Maicon, clearance by Frazer Richardson.85:53
The ball is sent over by Sisenando Maicon, save made by Artur Boruc.85:40
Aleksandar Kolarov sends in a cross.84:49
Foul by James Milner on Morgan Schneiderlin, free kick awarded. Jos Hooiveld takes the free kick.84:03
The offside flag is raised against Jay Rodriguez. Indirect free kick taken by Joe Hart.80:53
Unfair challenge on Rickie Lambert by Gael Clichy results in a free kick. Frazer Richardson takes the free kick.79:08 Substitution
Substitution Steven Davis leaves the field to be replaced by James Ward-Prowse.78:34
Corner taken by Aleksandar Kolarov from the left by-line, clearance made by Rickie Lambert.77:26
Unfair challenge on Frazer Richardson by Aleksandar Kolarov results in a free kick. Free kick taken by Maya Yoshida.75:56
The ball is swung over by James Milner, clearance by Maya Yoshida.75:15
Aleksandar Kolarov delivers the ball, save by Artur Boruc.73:06
Steven Davis concedes a free kick for a foul on Gael Clichy. Gael Clichy restarts play with the free kick.72:13 Substitution
Substitution David Silva goes off and Sisenando Maicon comes on.72:02
Jack Cork has an effort at goal from just inside the box that misses to the left of the target.69:33
Free kick taken by Maya Yoshida.
Substitution Frazer Richardson comes on in place of Daniel Fox.69:33 Booking
Booking Caution for Yaya Toure.69:28
Yaya Toure challenges Morgan Schneiderlin unfairly and gives away a free kick.68:43
The ball is delivered by James Milner, clearance made by Jos Hooiveld.68:22
Outswinging corner taken by James Milner from the right by-line played to the near post, Daniel Fox makes a clearance.68:13
The ball is sent over by James Milner, Jos Hooiveld makes a clearance.66:36
Sergio Aguero is ruled offside. Artur Boruc takes the free kick.65:25 Substitution
Substitution Aleksandar Kolarov is brought on as a substitute for Joleon Lescott.65:14
Morgan Schneiderlin has shot on goal from just outside the penalty box which goes wide of the right-hand upright.63:18
The referee blows for offside. Francisco Javi Garcia takes the indirect free kick.60:30 Substitution
Substitution Jason Puncheon goes off and Adam Lallana comes on.60:17
Yaya Toure produces a left-footed shot from outside the box and misses left.59:40
Jason Puncheon gives away a free kick for an unfair challenge on Gareth Barry. Yaya Toure takes the free kick.56:50
Free kick awarded for a foul by Yaya Toure on Steven Davis. Daniel Fox restarts play with the free kick.56:16
Centre by Jack Cork, Joe Hart makes a save.55:17
Corner taken by David Silva from the right by-line to the near post, clearance made by Morgan Schneiderlin.55:05
Corner taken by David Silva, clearance made by Jos Hooiveld.54:43 Substitution
Substitution James Milner replaces Samir Nasri.54:27
Sergio Aguero takes a shot. Save by Joe Hart.53:48
A cross is delivered by Daniel Fox, clearance made by Joleon Lescott.53:31
Steven Davis produces a cross.51:30
The ball is delivered by Daniel Fox.49:29
Free kick crossed left-footed by Daniel Fox from right wing, clearance made by Joleon Lescott.49:29 Booking
Booking Gael Clichy is booked.49:20
Gael Clichy concedes a free kick for a foul on Jason Puncheon.47:34
Assist on the goal came from Rickie Lambert.47:34 Goal scored
Goal! - Gareth Barry - Southamp'n 3 - 1 Man City Gareth Barry concedes an own goal. Southampton 3-1 Man City.46:46
Effort on goal by Jason Puncheon from just inside the penalty area goes over the target.45:54
Steven Davis takes a shot. Joe Hart makes a save.45:46
Edin Dzeko is penalised for handball and concedes a free kick. Free kick taken by Jos Hooiveld.45:01
The second half has started.45:00 +2:52 Half time
Half Time The half-time whistle blows.45:00 +2:33
The ball is delivered by Rickie Lambert, clearance by Pablo Zabaleta.45:00 +1:47
Corner taken by Daniel Fox.45:00 +1:09
Samir Nasri crosses the ball, David Silva takes a shot. Save made by Artur Boruc.45:00 +0:28
The ball is sent over by Steven Davis, Close range effort by Morgan Schneiderlin misses to the right of the goal.44:35
Jos Hooiveld gives away a free kick for an unfair challenge on Sergio Aguero. Free kick crossed right-footed by Samir Nasri from right channel, Jason Puncheon manages to make a clearance.44:22
Pablo Zabaleta fouled by Jay Rodriguez, the ref awards a free kick. Free kick taken by Samir Nasri.44:06
Jos Hooiveld gives away a free kick for an unfair challenge on Yaya Toure. Joe Hart takes the free kick.43:30
Francisco Javi Garcia concedes a free kick for a foul on Rickie Lambert. Daniel Fox takes the direct free kick.40:50
Corner taken by Steven Davis from the right by-line to the near post, Effort from the edge of the penalty box by Jason Puncheon goes wide of the right-hand upright.39:57
Corner taken by Daniel Fox from the left by-line, Maya Yoshida takes a shot. Pablo Zabaleta gets a block in.39:49
The ball is swung over by Rickie Lambert, clearance made by Francisco Javi Garcia.38:05
Assist by Pablo Zabaleta.38:05 Goal scored
Goal! - Edin Dzeko - Southamp'n 2 - 1 Man City Edin Dzeko fires in a goal from inside the six-yard box to the bottom left corner of the goal. Southampton 2-1 Man City.37:23
Outswinging corner taken by Steven Davis from the right by-line.37:11
Nathaniel Clyne takes a shot. Joleon Lescott gets a block in.34:03
Free kick awarded for an unfair challenge on Nathaniel Clyne by Yaya Toure. Maya Yoshida restarts play with the free kick.33:13
Shot from just outside the box by Pablo Zabaleta misses to the left of the goal.33:05
The ball is delivered by David Silva, Jos Hooiveld manages to make a clearance.31:57
Effort from deep inside the area by Jay Rodriguez misses to the right of the target.31:40
Jason Puncheon takes a shot. Blocked by Gareth Barry.31:09
Corner from the right by-line taken by Steven Davis, Yaya Toure makes a clearance.29:04
Rickie Lambert is ruled offside. Free kick taken by Joe Hart.27:20
Free kick awarded for a foul by Jos Hooiveld on Edin Dzeko. Sergio Aguero fires a strike on goal direct from the free kick.26:53
Joleon Lescott takes the direct free kick.26:53 Booking
Booking Rickie Lambert goes into the book for unsporting behaviour.26:48
Free kick awarded for an unfair challenge on Joleon Lescott by Rickie Lambert.25:49
The offside flag is raised against Rickie Lambert. Joleon Lescott takes the indirect free kick.25:37
The ball is crossed by Gael Clichy, Jos Hooiveld makes a clearance.24:25
Jay Rodriguez is ruled offside. Joleon Lescott restarts play with the free kick.23:50
The referee blows for offside against Rickie Lambert. Free kick taken by Joe Hart.21:33
Assist on the goal came from Rickie Lambert.21:33 Goal scored
Goal! - Steven Davis - Southamp'n 2 - 0 Man City A goal is scored by Steven Davis from inside the six-yard box low into the middle of the goal. Southampton 2-0 Man City.21:29
Rickie Lambert takes a shot. Save by Joe Hart.21:10
Free kick awarded for an unfair challenge on Jay Rodriguez by Pablo Zabaleta. Daniel Fox takes the direct free kick.20:56
The referee blows for offside. Indirect free kick taken by Maya Yoshida.20:46
David Silva crosses the ball, clearance by Daniel Fox.20:19
Corner taken by David Silva played to the near post, Daniel Fox manages to make a clearance.20:08
The ball is swung over by Pablo Zabaleta, Nathaniel Clyne manages to make a clearance.19:02
A cross is delivered by Pablo Zabaleta, clearance made by Jos Hooiveld.17:55
The ball is delivered by Daniel Fox, Gael Clichy manages to make a clearance.17:16
Maya Yoshida fouled by Edin Dzeko, the ref awards a free kick. Free kick taken by Artur Boruc.15:59
Jason Puncheon challenges David Silva unfairly and gives away a free kick. David Silva restarts play with the free kick.15:33
Sergio Aguero sends in a cross.14:43
Jay Rodriguez is adjudged to have handled the ball. Joe Hart takes the direct free kick.14:04
Pablo Zabaleta is ruled offside. Artur Boruc takes the free kick.13:34
Samir Nasri delivers the ball, Daniel Fox gets a block in.10:15
Corner taken by Steven Davis played to the near post.10:09
Nathaniel Clyne sends in a cross, Joleon Lescott manages to make a clearance.8:08
Sergio Aguero takes a shot. Save made by Artur Boruc.6:22
Jay Rodriguez provided the assist for the goal.6:22
Assist by Jay Rodriguez.6:22 Goal scored
Goal! - Jason Puncheon - Southamp'n 1 - 0 Man City Jason Puncheon grabs a goal from just inside the area to the bottom left corner of the goal. Southampton 1-0 Man City.6:04
Maya Yoshida gives away a free kick for an unfair challenge on Sergio Aguero. Direct free kick taken by David Silva.5:23
Edin Dzeko is adjudged to have handled the ball. Daniel Fox restarts play with the free kick.1:44
Unfair challenge on Rickie Lambert by Pablo Zabaleta results in a free kick. Daniel Fox takes the free kick.1:20
Morgan Schneiderlin fouled by Gareth Barry, the ref awards a free kick. Maya Yoshida takes the free kick.0:00
The ref blows the whistle to start the match.
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