A match officials' union has called for Sir Alex Ferguson to retract his recent remarks about referee Martin Atkinson. The Manchester United chief was angered by Atkinson's decision not to give Chelsea's David Luiz a second yellow card in Tuesday night's 2-1 defeat at Stamford Bridge.

Alan Leighton, the national secretary of Prospect, a trade union representing match officials, said: "The idea you can have referees who are biased towards one side or another, at any level, is one that causes great damage to the game," said Leighton.

"Referees will make mistakes but they make them because they are human beings. It is absolutely fair for managers to say referees make mistakes.

"If any referee couldn't accept that they wouldn't have got past park football.

"But where the dividing line between legitimate and illegitimate criticism comes is when you start to question the integrity of the officials or say they are not fit to do their job, either physically or because they are biased.

"By saying they didn't have a fair referee, Sir Alex Ferguson is saying that Martin Atkinson was favouring one team or another.

"If you start to let those comments go, it is not far beyond for other managers to think they can also query the fairness of referees.

"If Sir Alex didn't mean that it would be very simple for him to say so and it would clear the issue up. The sensible thing to move this on would be an apology."

Ferguson will find out by 4pm on Friday if he is to be charged with improper conduct by the FA.