Metropolitan police act on Society of Black Lawyers 'complaint'Clattenburg set to formally deny Chelsea's allegations Official understood to be angry and shockedChelsea still to decide over making formal complaint to FA

Mark Clattenburg will formally deny allegations he used "inappropriate language" towards the Chelsea players Mikel John Obi and Juan Mata, with the Metropolitan police now having launched its own investigation.

It is understood Clattenburg has been shocked and angered at being accused by Chelsea of using comments that have been interpreted as racist during their defeat by Manchester United on Sunday.

According to reports, Clattenburg is alleged to have called Mikel a "monkey" and Mata a "Spanish twat", although the 37-year-old official rejects the allegations.

But on Tuesday night it emerged that Chelsea were still considering whether to press ahead with an official complaint to the Football Association, with the club aiming to conclude their internal investigation before Thursday with the aid of outside legal counsel.

It has emerged that despite Chelsea on Sunday drawing the Premier League match delegate's attention to their allegations regarding Clattenburg, a formal complaint was not lodged by the club with the FA.

Once Chelsea have taken all relevant statements from players and staff through their secretary, David Barnard, they will decide whether to contact the FA over the matter for the first time. The governing body in the meantime has been making inquiries on the back of the match delegate's report.

The Guardian understands the FA is yet to speak to the referee about the matter, or to United to establish if any of their players may be potential witnesses.

With United due back at Stamford Bridge on Wednesday evening for a potentially combustible Capital One Cup tie, Oriol Romeu, an unused Chelsea substitute on Sunday, appeared to confuse the picture of what may have happened when he told a Spanish radio station, Cadena COPE: "Neither Juan nor Fernando [Torres] told me they heard it because from what I have understood they didn't hear anything.

"It was someone else who heard it, but not directly at them. It was another player who heard it and that is what they told me. I didn't hear anything as I left quickly to go home, but after talking to Juan he told me there was some problem and he had to stay.

"I only know Chelsea made a complaint and I think there could really be a problem if what Chelsea players say happened really happened. If there was really a racist comment or something said against a Spanish player this will be serious. We know in this country people are very vigilant about these issues."

The Metropolitan police confirmed it is seeking to establish whether any charge should be brought following what it described as a "complaint" made by the Society of Black Lawyers through its chairman, Peter Herbert.

A statement from the force read: "An investigation has been launched into alleged comments made during a football match between Chelsea FC and Manchester United FC at Stamford Bridge on 28 October 2012. This follows on from a complaint received by the Metropolitan police service on 29 October. Officers from Hammersmith & Fulham borough are in liaison with Chelsea football club and the Football Association.

"At this time, the Met has not received any complaint from either Chelsea football club, or the Football Association. We continue to work in partnership with Chelsea football club and the Football Association in order to consider any allegation that is made in relation to the reported events."

Herbert defended his decision to involve the police. "What we don't want is for it to be swept away under the carpet," he told Sky Sports News. "It must be subject to a full and proper investigation. It is to lend some seriousness and some weight behind what is happening in football."

Herbert admitted his complaint was based on reports rather than first-hand evidence, but added: "We weren't there but we don't need to be there in order to report an incident. This appears to have had some cogency and so it needs to be addressed as a matter of urgency. I think the Met police have huge resources, expertise, and I have no doubt that this matter will be resolved and the truth will come out. If we've got this completely wrong then, of course, the police will tell us."

Rio Ferdinand, whose brother Anton Ferdinand was last year racially abused by John Terry, the Chelsea captain, reminded the FA of the scrutiny it is under regarding its handling of the allegations against Clattenburg.

In a tweet he wrote: "Will b interesting t see how this alleged 'race' or 'insulting' language episode pans out, our game is under the microscope like never before."