Three defensive howlers from Manchester City helped Southampton romp to a 3-1 win at a rocking St Mary's Stadium.
Jason Puncheon and Steven Davis took advantage of errors from Gareth Barry and Joe Hart to put the Saints 2-0 up after 23 minutes.
Edin Dzeko gave City hope shortly before half-time but Barry gave fresh meaning to the term 'comedy own goal' with a lethal side-foot into his own net two minutes into the second half.
What larks for Southampton, who sit six points clear of the relegation zone. What a shambles from the champions, whose title defence has almost completely disintegrated.
Manchester United will surge 12 points clear of their second-placed rivals if they beat Everton at Old Trafford on Sunday afternoon.
The inquest into this February farce promises to be a painful one for City, who lacked organisation, tenacity, purpose and any of the qualities expected of a team trying to make up ground in the title race.
They were completely out-played by Southampton, whose relentless pressing and attacking intent was vindication for the work of new manager Mauricio Pochettino.
Moreover, the Argentine was without brilliant teenage left-back Luke Shaw, who missed out with ligament damage suffered while training with the England Under-21 side this week.
Yaya Toure returned from African Cup of Nations duty to captain the champions, while Samir Nasri came in for James Milner but one player City desperately could have done with was wearing a smart suit rather his playing kit.
Vincent Kompany travelled with the City party and was watching from the stands but he was still not fit to return to the 18-man squad.
How Mancini must have lamented the absence of his talismanic skipper, firstly when Southampton unlocked the re-jigged City defence in the seventh minute.
Jason Puncheon used his pace to wriggle clear on the right, threaded a neat ball through to Jay Rodriguez, whose shot was well parried by the alert Joe Hart. However, the loose ball fell to Puncheon, who tucked in his seventh goal of the season.
City struggled to gain a foothold in the match and a goalkeeping howler midway through the opening period gifted the hosts their second.
Hart's dip in form has coincided with a national television advert in which he promotes a shampoo, and the entire City staff were left tearing their hair out when he allowed a Rickie Lambert shot headed for his midriff to squirm out of his grasp, between his legs and into the path of the grateful Steven Davis, who tucked it in.
City had been desperately poor for most of the first half but there was no doubt about the quality of the goal which signalled a comeback could be on.
After Dzeko had headed clear a Southampton corner, he raced up the other end to finish off a counter-attack that had been beautifully worked until Pablo Zabaleta's cross was squared behind the Bosnian. Dzeko slipped as he checked his run but somehow managed to get a foot on the ball to stab in his 12th league goal of the season.
Shortly after Puncheon had blazed over the bar, the home side gained further reward for their purpose and attacking intent following a brain explosion from Barry.
The lively Lambert broke free down the left but his cross should have been comfortably dealt with by Barry, who, under no pressure at all, inexplicably passed the ball into the bottom corner of the net.
Most observers expected some sort of reaction from City, a late assault on the hosts' goal or the kind of sustained comeback for which they have become renowned over the last two seasons.
But it never really materialised. Southampton drew the sting from the visitors with relative comfort and David Silva was mysteriously removed from the fray by Mancini with 17 minutes remaining.
It was an odd decision on an evening of perverse decisions from City employees.