Ref called Mikel monkey — Chelsea.

The race-row that has engulfed Premier League referee Mark Clattenburg intensified further on Wednesday as two Chelsea players claimed they heard him call teammate Mikel Obi a ‘monkey’.

It has emerged that the pair of unnamed stars say this is what happened during their match with Manchester United on Sunday — but the official categorically denies making any racist comments.

Clattenburg has been accused of saying to Mikel, ‘Shut up you monkey’ as he booked him for dissent as well as calling Juan Mata a ‘Spanish t***’ in their 3-2 defeat at Stamford Bridge.

Mystery had surrounded what Clattenburg actually said to Mikel – until these allegations were made Wednesday morning.

But a source close to the referee told The Sun, “He said nothing of the sort – and he will fight this all the way.

“There is a lot of noise on the pitch and the players could have easily misheard.”

Meanwhile, Chelsea’s race case against Clattenburg is in serious danger of collapsing completely after it emerged that the club have employed independent lawyers to investigate the claims of their players.

The club are expected to conclude their internal investigation into allegations that the referee racially abused Mikel and swore at Juan Mata.

The external lawyers will then provide Chelsea with legal opinion as to whether their case will be successful. Despite issues over the available evidence, Chelsea remained robust.

Chelsea will proceed with the case against the referee only if they are satisfied the claims made by the players after the game will stand up to the FA’s burden of proof.

It means they will need to satisfy an FA commission ‘on the balance of probability’, but they also have to negotiate the complexities of the Metropolitan Police’s investigation into the affair.

If the case breaks down, it will reflect badly on Chelsea, who made their claims of inappropriate language against Clattenburg in a strongly worded statement on Sunday, two hours after the final whistle.

Earlier it emerged that the club and Mikel could be charged with misconduct by the FA after the Nigerian had to be restrained from manhandling the referee in his dressing room following the clash.

Clattenburg did not mention the behaviour in the ‘extraordinary incident’ report he filed on Sunday, but he now has to make a detailed statement to the FA. Clattenburg’s assistants, Michael McDonough and Simon Long, and fourth official Mike Jones have filed reports in which they say they did not hear the Durham official say anything inappropriate.

All four were in the referee’s room when Mikel burst in with Chelsea chief executive Ron Gourlay and manager Roberto Di Matteo.

Mikel’s aggressive manner, along with the behaviour of other Chelsea officials, raises the possibility of FA action against the club.

Both Mikel and Mata claim they have a witness to Clattenburg’s comments, and PFA chief executive Gordon Taylor, who has assessed both complaints, told Sportsmail the pair have his ‘100 per cent support’.