Gareth McAuley struck a dramatic late winner as West Bromwich Albion defeated Chelsea 1-0 to heap the pressure on Blues boss Andre Villas-Boas on Saturday.
Northern Ireland international McAuley slid in to convert from close range with just eight minutes remaining at The Hawthorns to leave Chelsea three points adrift of the fourth and final Champions League spot following Arsenal's victory over Liverpool in the day's early game.
While it was a third straight victory for West Brom, Chelsea have now won just one of their last seven matches and the defeat will only add to the uncertainty over Villas-Boas' future, who conceded this week he fears for his job at Stamford Bridge.
Chelsea return to the midlands in midweek for an FA Cup fifth round replay against Birmingham and a defeat at the hands of the Championship club could prompt Blues owner Roman Abramovich to sack Villas-Boas.
Once again, there was no place for Fernando Torres in Chelsea's starting line-up, yet Frank Lampard kept his place despite last week's admission that his relationship with his manager was "not ideal".
Roy Hodgson's side dominated the early stages and threatened in the eighth minute when James Morrison forced Petr Cech to tip a swerving 25-yard effort bound for the top corner over his crossbar after Jerome Thomas' lay-off.
Chelsea's stuttering start was summed up when the unmarked Juan Mata completely fluffed his lines 15 yards out from Daniel Sturridge's cross, however the Spaniard did better in the 14th minute when Didier Drogba's flick gave him sight of goal from a tight angle only for him to be foiled by Ben Foster.
On-loan Foster's recent form has prompted Hodgson to admit it could price them out of a permanent deal in the summer and the goalkeeper demonstrated he is at the top of his game with a fine fingertip save to keep Sturridge's volley from the top corner after 39 minutes.
However, the England forward spurned an excellent chance a minute later when he was sent through by Drogba's threaded pass only to drag a shot past the post.
Albion threatened again shortly before the interval, though, and it took a fortunate deflection off Gary Cahill to stop Youssouf Mulumbu's curling effort giving the Baggies the lead.
Foster excelled once more when he darted off his line and emerged victorious from a one-on-one confrontation with Sturridge outside of his own area eight minutes after the restart.
Mata capitalised when Steven Reid pulled up with an injury and the Spaniard set Sturridge clean through on the counter-attack, however Foster read the former Manchester City player's dummy and the danger was averted.
Reid was forced off, with Gabriel Tamas replacing him, and he was soon followed onto the field by Baggies captain Chris Brunt for his first appearance since a lengthy absence through injury.
Mulumbu and Marc-Antoine Fortune both forced smart saves from Cech, while Brunt let fly with a low drive as the Baggies continued to knock on the door.
Villas-Boas finally responded to Chelsea fans' calls for Torres by introducing him in the 76th minute in one last throw of the dice.
Yet it was the Baggies who were to eventually prosper. Cech could only punch Brunt's deep corner out to Keith Andrews, wide on the left.
Andrews' cross was fired back across goal by Liam Ridgewell and McAuley slid in to spark wild celebrations among the Albion fans.