Leeds manager Neil Warnock has said he is 'disgusted' with the conduct of Chelsea's players and has backed referee Mark Clattenburg as the Stamford Bridge race row intensified.
Warnock has accused Chelsea of 'trying to kill' the referee and urged the FA to throw the book at players including John Obi Mikel, if their racism claims prove to be false.
The Leeds manager is no stranger to controversy - and you might think he would need to focus on his own club after a fan attacked Sheffield Wednesday goalkeeper Chris Kirkland.
But the outspoken Warnock turned his attention to Chelsea's players.
'You know my relationship with referees but I have got to say I am disgusted with what's gone on,' said Warnock. 'I'm on Mark Clattenburg's side.'
In the spotlight: John Obi Mikel leaves Chelsea training on Tuesday as the row hangs over the club
Referee Mark Clattenburg is now at centre of a police investigation over allegations that he racially abused John Obi Mikel.
There are now fears that the controversy will be left unresolved for months like the John Terry and Anton Ferdinand race row.
PFA chief executive Gordon Taylor has already warned that the issue must not be allowed to 'fester'.
It comes after claims in the Daily Mirror that Clattenburg was threatened by a member of the Chelsea squad, who told him: 'I'm going to f****** break your legs,'.
It follows claims that Clattenburg, who is the subject of an FA and police investigation, racially abused Nigerian midfielder John Obi Mikel. Clattenburg denies the allegation and he is supported by his two assistants, Michael McDonough and Simon Long.
Professional Footballers' Association chief executive Taylor said: 'Involving police or waiting causes a massive festering of the issue, which has continued to cause problems and is not good for the image of the game.
The Metropolitan Police started their probe after an official complaint from the Society of Black Lawyers. The complaint was made on Monday, although the group admit it is on the back of media reports rather than first-hand evidence.
A police statement 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.
All smiles: Mikel was back with his team-mates in training amid the race row
Eyes on the ball: Juan Mata (centre) and John Terry (right) during training on Tuesday morning
'At this time, the MPS 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.'
It was unclear whether Clattenburg would elect to continue refereeing or take a break from the game while any investigation was pending.
Police became embroiled in a second high-profile football racism case after the man behind the mooted black players' breakaway union, Peter Herbert, sent a letter to the Metropolitan Police Service (MPS), which read: 'The Society of Black Lawyers wishes to record the incident that took place at Chelsea FC.
'Our information is that racist remarks were directed at John Mikel Obi and at Juan Mata. If so, that is wholly unacceptable in any circumstances. Although this matter may be investigated by the FA, it is appropriate that the MPS independently see if a racially-aggravated offence has occurred.
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.'
Focus: Juan Mata drives into Chelsea's Cobham HQ ahead of Wednesday's clash with United
At the wheel: Fernando Torres was also among the players training on Tuesday morning
According to the Daily Mirror, players and staff are claimed to have swarmed into the referee's room at Stamford Bridge in the aftermath of the explosive Barclays Premier League game with Manchester United when a heated exchange took place, resembling a 'pub brawl'.
The Mirror claims Chelsea players and officials 'charged into the room and acted like bouncers'. The threat to Clattenburg was heard coming from within the room.
The Mirror also reports that Roberto Di Matteo left the room 'drenched in sweat' and that captain John Terry, who was suspended from the game following his FA charge for racially insulting Anton Ferdinand, was close to the fracas. However, sources close to Terry deny he was involved and, in fact, insist he 'stayed well clear.'
After the Daily Mirror claims emerged, reporter Darren Lewis told Sky Sports News: 'There were lots of things that were said in what was an explosive confrontation in the referee's room after the match.
'That ("I'll break your legs") is one of the things that has allegedly been said. It's not been ascertained by whom but a lot of things have been said inside the heat of the dressing room during a furious confrontation.'
Among those who were either in or close to the referee's room were manager Roberto Di Matteo and chief executive Ron Gourlay, who were thought to be acting as peacemakers, trying to calm the angry Mikel.
It is reported that 'banging and shouting' could be heard from inside the room and security staff were later involved.
Clattenburg has filed an 'extraordinary incident' report into the matter and has been stood down from Saturday's matches while the investigations into his conduct take place.
Manchester United's players, who won 3-2 at Stamford Bridge against nine-man Chelsea, are expected to be asked by the FA: 'What did you hear on - and off - the pitch'.
When Obi Mikel stormed into the referee's room on Sunday evening, he believed he had every right to be angry with Clattenburg.
Chelsea had made a stand, wearing Kick It Out's â€˜One Game One Community' T-shirts before one of the most prestigious fixtures in the Barclays Premier League.
It was a fresh start, with handshakes all round and an anti-racism badge pinned to John Terry's jacket on the sidelines as he continued to serve a four-match ban for insulting Anton Ferdinand.
What followed is in danger of dragging the game back into the gutter, sparking bitter recrimination and - for the first time in the professional game's history in England - an allegation of racism against a referee.
Mikel responded after team-mates David Luiz and Ramires told him they believed he had been racially abused by the FIFA official moments after he was booked at Stamford Bridge.
Mikel did not hear the alleged abuse, but Ramires and Luiz are prepared to make statements saying they did.
Trouble: John Obi Mikel stormed into Mark Clattenburg's room
Centre of the storm: Referee Mark Clattenburg and Chelsea midfielder John Obi Mikel
Shocked: Juan Mata is unhappy with Clattenburg's alleged insult
In the explosive scenes that followed the final whistle, Di Matteo and Gourlay had to stop Mikel attacking Clattenburg during the confrontation.
Heated conversations between Mikel and the referee had begun when Fernando Torres was sent off after being booked for â€˜aggressive simulation'.
The central midfielder continued to cross swords with Clattenburg during the next few frantic minutes as he demanded an explanation for the decision.
Mikel wanted to know why Torres had been booked for a second time when Chelsea's players were convinced he had been caught by Jonny Evans on his way through on goal in the 68th minute. When Clattenburg told Mikel to keep quiet and to get on with the game, the Nigeria midfielder continued to protest.
John Terry watches from the stands as Ashley Young takes on Juan Mata
It is understood Mikel used words to the effect of: â€˜Why can't we stand up for ourselves, why can't we challenge your decisions?'
The dismissal of Torres had reduced Chelsea to nine men, coming on top of the 63rd-minute red card for Branislav Ivanovic, and emotions were running high.
When Torres was sent off, Di Matteo and Sir Alex Ferguson clashed on the touchline as coaching staff from both clubs spilled into the technical area. Clattenburg appeared to have the game under control again until the moment Javier Hernandez scored United's winner from an offside position in the 75th minute.
As United celebrated near the corner of the West Stand and Matthew Harding Stand, some Chelsea supporters threw coins and a seat on to the field.
Clattenburg soon restored order but Mikel was in his ear again and making the same complaints, saying the decision to send off Torres was the reason Chelsea would lose the game.
In the blue corner: Javier Hernandez celebrated his goal in front of the Chelsea fans
Clattenburg booked him for dissent in the 76th minute and it is at this moment that Ramires and Luiz allege they heard the midfielder being racially abused by a referee who is known for having a dismissive and patronising tone when he talks to players about their behaviour.
Mikel was immediately told about the allegation, but decided to wait until after the game to confront Clattenburg with the claims.
Chelsea have also confirmed a separate incident, understood to involve Juan Mata, after claims that Clattenburg called him â€˜a Spanish t**t'.
Mata is an astute and intelligent professional and, according to sources at the club, he was visibly shocked when he heard Clattenburg's insult.
Chelsea's position had not changed on Monday night and the incident was the talk of their Cobham training ground in Surrey during the day. Many of the players were in for a light session after the 3-2 defeat against United and they eagerly discussed Clattenburg's behaviour.
Standing alone: Clattenburg found few friends in Stamford Bridge
Some of them will face United again in the Capital One Cup at Stamford Bridge on Wednesday night as Chelsea prepare for another lengthy spell in the spotlight.
Although the players are ready for another FA investigation, it is understood that Manchester United are unable to help with their enquiries.
Most of their players have been asked if they heard - or were made aware of - either incident, but they are unable to provide any assistance.
Mikel has since told team-mates he intends to pursue the matter and they have agreed to provide statements to the FA if asked.
Following the angry scenes in the referee's room, witnessed by Clattenburg's assistants Michael McDonough and Simon Long, Chelsea discussed the incident at length. Mikel was taken away from the corridor that leads to the tunnel so the media, separated from the dressing-room areas by the width of a wall, could not hear what was said.
During those discussions, Gourlay decided to press ahead with a complaint to match delegate Nick Cusack and arranged for some of the details to be made public at 7.45 that evening.
Chelsea's deliberations were the reason for the delay between the final whistle and Di Matteo's appearance at his post-match press briefing.
In the spotlight: Mark Clattenburg leaves his home near Newcastle on Monday
Clattenburg, as required by the FA, then filed details of an â€˜extraordinary incident' at Stamford Bridge overnight.
On Monday morning he received communication from the FA's compliance team as they prepared for a swift investigation into the remarkable allegation.
Chelsea are also under intense scrutiny after being criticised by the FA's independent regulatory commission following the commission's decision to ban Terry for four games and fine him Â£220,000 for racially abusing Anton Ferdinand.
The commission judged that the Chelsea skipper's evidence relating to the incident at Loftus Road in October 2011 was â€˜improbable, implausible and contrived'.
They also reflected on the â€˜evolution of Ashley Cole's evidence' that led to an infamous tweet directed at the FA for which he was fined Â£90,000, and concluded that the evidence given by long-serving club secretary Dave Barnard was â€˜materially defective'.
After Terry was banned by the FA, Di Matteo encouraged the public to give the disgraced captain â€˜a second chance'.
Clattenburg could be charged under Rule E3, the same as Terry, and there is a determination within the FA for a swift resolution to yet another crisis.