He also suggested the Tyne and Wear official had been inconsistent in deciding to dismiss the Scotland international, citing his lenient handling of Arsenal's derby with Tottenham earlier this season. 'You had to hit someone with an axe before [Clattenburg] would book anyone,' he added.
The FA have confirmed that they are investigating the comments. 'The FA have noted Sir Alex Ferguson's comments and will consider whether this is a matter for their governance amd disciplinary department today,' a spokesman said.
Ferguson has a two-match suspended touchline ban hanging over him for criticising the fitness of referee Alan Wiley after the 2-2 draw with Sunderland in October. If his latest remarks are deemed to constitute a personal attack on Clattenburg, he could be banned from the dugout for two games, although reports last night suggested he could escape sanction.
Clattenburg last defended his decision. 'In the first half he [Fletcher] was b-----ked for dissent; I warned him for dissent,' he said. 'Then he was cautioned before half-time for a foul. Then he cynically tripped the Birmingham player.'
Meanwhile, United striker Dimitar Berbatov could be forced to undergo an exploratory knee operation after he pulled out of the game at St Andrews. Berbatov has been struggling with the problem for several weeks and Ferguson admitted: 'He wanted to play but we felt we should try and get it sorted. He may have to have an exploratory operation.'
United last night flew to Qatar for a warm weather training camp. They will return on Thursday, 48 hours before their home game with Burnley.