Sam Allardyce described Chelsea's winning goal in their 2-1 victory over Newcastle as a 'clear offside'.
The under-pressure Newcastle manager saw his players put in a fine performance at Stamford Bridge, only to lose to Salomon Kalou's goal five minutes from time.
The Ivory Coast striker appeared to be in an offside position when the ball deflected off Claudio Pizarro into his path, but the linesman's flag stayed down.
It proved to be a costly decision for Newcastle, who cancelled out Michael Essien's opening goal when Wayne Bridge touched the ball into his own net under pressure from Nicky Butt.
"We were very good today and we did all of it right. Unfortunately for us the game has been taken away by something that we can't control," Allardyce told Sky Sports.
"It's not a decision that can be judged in terms of 'it was close' it was just a major decision that went against us - a clear offside by two or three yards.
"The ball hit Pizarro I think it was and dropped to a Chelsea player in an offside position who put it in the net. You don't know why an assistant referee can't see across that line and see that player in that position."
After a dismal draw at home to Derby and the Boxing Day defeat at Wigan, Newcastle had caused plenty of problems for Chelsea and were desperately unlucky to lose the game.
Obafemi Martins missed two good chances early in the first half, and after the equaliser the visitors threatened to take the lead on a couple of occasions.
"For us it's a huge blow to our confidence after a tremendous performance which should have got us at least a point," Allardyce said.
"We could maybe have scored a second in the second half and of course we had two clear chances for Oba in the first one-on-one."