After London rivals Tottenham had lost at Fulham on Tuesday, it was a golden opportunity for the Gunners to put themselves right back in the frame for a top-four finish.

However, after a promising start, Arsenal were undone by some more poor defending, as Sol Campbell’s mistake let in Nigel Reo-Coker to net the opening goal on 25 minutes.

Former Spurs man Bobby Zamora soon made it 2-0, and although Thierry Henry poached his club-record 151st league goal, and Robert Pires later set up a grandstand finish, Matthew Etherington’s 81st-minute effort sealed all three points.

It had been a promising start to the match by Arsenal, looking to bounce back from their disappointing double cup exit last week, and they should have been ahead on six minutes.

Abou Diaby made ground down the left, taking the ball into the area and to the goal-line.

His cut-back picked out Henry on the penalty spot, only for the captain’s side-foot effort to strike team-mate Freddie Ljungberg.

Robin Van Persie then dashed into the left side of the box before turning Danny Gabbidon inside out and then lashing an angled drive against the post from close range.

Arsenal goalkeeper Jens Lehmann finally had something to do when he got down to save Yossi Benayoun’s weak strike from 18 yards.

Then, out of nothing, West Ham went ahead on 25 minutes.

Hayden Mullins’ quick throughball should have been cut out by Campbell, but the veteran England defender made a complete hash of his attempted clearance.

Reo-Coker gratefully latched on to the loose ball, sprinted clear and coolly slotted past Lehmann.

Arsenal looked to have a strong penalty claim when Paul Konchesky appeared to handle a deep cross from the left by Senderos but referee Mark Halsey was unimpressed.

Arsenal’s night got worse as Zamora made it 2-0 on 32 minutes. A long, diagonal ball from Konchesky was controlled expertly by the former Spurs man on his chest as he moved into the penalty area.

Zamora then held off Campbell’s challenge before dispatching a fine left-footed effort into the top corner.

Johan Djourou’s goalbound effort was hacked off the line by Benayoun before Arsenal pulled one back on the stroke of half time.

Following Ljungberg’s charge into the box, Van Persie smashed the loose ball goalwards, which Benayoun again was on hand to clear.

Robert Pires hit a low strike on target from the rebound, and Henry toe-poked in for his 151st league goal for Arsenal, breaking Cliff Bastin’s record.

Campbell was not given the chance to redeem himself in the second half, Wenger deciding to replace the veteran centre-back with young Swede Sebastian Larsson for his first taste of Premiership action.

On 58 minutes Ljungberg burst down the left channel and forced Hislop into a fine block, standing up well at his near post.

Ferdinand then upended Henry just outside the area as the Arsenal captain looked to spin clear.

Van Persie’s free-kick was whipped around the far side of the wall, and fortunately for the Hammers, Gabbidon’s attempted flick took the ball the other side of the post.

With 25 minutes left, Henry made some space for himself on the left side of the area and flashed a low effort across the face of goal.

West Ham’s record £7m signing Dean Ashton made his debut for the final 18 minutes, replacing Zamora, but it was at the other end where the Hammers needed help relieving the pressure.

But the visitors then made sure of a first league win in nine attempts when Matthew Etherington’s effort deflected in off Flamini, Pires unable to keep it out on the line.

At the other end Pires smashed in a late effort to set up a grandstand finale, following Hislop’s save from substitute Dennis Bergkamp.

But it was too little, too late for Arsenal, who now face an uphill battle to save their season.