Chelsea suffered a major blow to their title challenge as West Ham inflicted their first home League defeat since April to end what could prove to be a defining week in the Blues' season.

A scrambled effort from Paul Kitson 14 minutes from time was enough to give the Hammers their first away win since November 22.

Knocked out of the FA Cup in their quarter-final replay with Manchester United in midweek, Chelsea found themselves on the receiving end of an even more damaging defeat against their London rivals.

Blunt in attack, they failed to capitalise on a number of chances in the first hour as the Hammers produced a stirring rearguard action in which Rio Ferdinand excelled, and the visitors even had one effort ruled out on the stroke of half-time.

The importance of this defeat, on a day when close rivals Arsenal and Manchester United both won tricky games away from home at Everton and Newcastle respectively, cannot be under-estimated.

Chelsea now stand seven points behind leaders United, even though they still have a game in hand, but with back-to-back defeats within the space of four days, their determination, resilience and experience will now be tested to the limit.

For after that defeat at the hands of United, Chelsea simply had to bounce back with a victory against the Hammers, with only 10 games of their Premiership season now remaining.

They were given hope by the absence of Paolo Di Canio, whose injury prevented his first meeting with fourth official Paul Alcock since his infamous push on the referee last September.

But the visitors set their stall out early on, with Kitson up front on his own and a five-man midfield from which Marc-Vivien Foe dropped back to shield the defence.

Chelsea should be used to such tactics from their European experiences but for the first half-hour they were unable to find the inspiration to break down the Hammers and create a single clear-cut chance.

It was 33 minutes into the game when Tore Andre Flo headed down Celestine Babayaro's searching cross into the path of Gianfranco Zola, but the Italian scuffed his shot and it was deflected wide.

Babayaro himself was then left in even more space shortly afterwards when Bjarne Goldbaek's long cross picked him out at the far post but his finishing was just as weak, and Ferdinand cleared his mis-hit volley off the line.

With Hislop tipping a Roberto Di Matteo drive over and diving low to gather a Zola free-kick, West Ham's best chance was a solitary effort on the break from Trevor Sinclair, which dipped a foot over the bar.

That was until the controversy which surrounded Foe's disallowed `goal' just two minutes before the interval.

The Cameroon international blasted the ball home from close range but referee Steve Lodge immediately disallowed it and awarded a free-kick to the home side after spotting pushing in the scrambled build-up which had ensued from acorner.

The visiting supporters roundly booed the official off the pitch at the break but again found their team under siege at the start of the second period.

However, Chelsea still lacked the creativity to find the breakthrough, with Hislop saving a snap-shot from Zola and tipping Goldbaek's deflected strike around the post before the Danish midfielder unwittingly blocked an effort from Flo.

With Chelsea - already without the injured Frank Leboeuf - replacing Marcel Desailly with Michael Duberry and Le Saux with Dan Petrescu - the Hammers started to sense an unlikely victory.

They bravely threw the impressive Sinclair up front alongside Kitson and were rewarded for their increased sense of adventure 14 minutes from the end. Di Matteo conceded a free-kick out wide on the Chelsea right, Marc Keller swung in a dangerous cross and Foe headed it downwards.

Kitson spun on the ball three yards out and although De Goey's diving body took some pace off the ball, it continued to roll goalwards and despite the efforts of several Chelsea defenders to clear it, referee Lodge deemed that it had crossed the line.

The goal sparked isolated outbreaks of violence within the crowd, which were quickly dealt with by police and stewards, but Chelsea were clearly in deep trouble.

The Blues threw on teenager Mikael Forssell for Andy Myers with seven minutes left and moved to a back three but still they lacked the finishing touch to their endeavours.

Forssell headed Goldbaek's cross just wide and Wise blazed wildly over but as Alcock signalled two minutes of injury time, it was West Ham who broke clear only for De Goey to dive out to cut out Foe's dangerous cross into the box.

At the final whistle, there were jubilant scenes among the Hammers' fans and players as Chelsea dejectedly trooped off.

They will rue their missed chances but if United received the credit for blunting Chelsea's attack with determined defending and scoring from a handful of chances as they did midweek, then so should West Ham.

Teams:

Chelsea: De Goey, Ferrer, Desailly (Duberry 60), Myers (Forssell 83), Le Saux (Petrescu 60), Goldbaek, Wise, Di Matteo, Babayaro, Flo, Zola.

Subs Not Used: Hitchcock, Morris.

Booked: Duberry, Petrescu.

West Ham: Hislop, Keller (Potts 86), Pearce, Ferdinand, Ruddock, Minto, Lampard, Foe, Lomas, Kitson, Sinclair.

Subs Not Used: Moncur, Lazaridis, Berkovic, Forrest.

Booked: Foe, Keller.

Goals: Kitson 75.

Att: 34,765

Ref: S Lodge (Barnsley).