Sir Alex Ferguson has poured scorn on Chelsea's claim Mark Clattenburg used "inappropriate language" towards John Obi Mikel, declaring: "I don't believe it."
But Blues boss Roberto Di Matteo insisted the allegation "should be treated seriously, no matter what", adding of Manchester United manager Ferguson: "He likes to talk about other clubs."
Ferguson has finally had his say on the latest race row to rock football, and he did not pull his punches, insisting there was "no way" referee Clattenburg would have abused Chelsea midfielder Mikel during United's stormy Barclays Premier League win at Stamford Bridge on Sunday.
Police and the Football Association launched formal investigations into the European champions' allegations Clattenburg used "inappropriate language" towards Mikel, something Press Association Sport understands the official strenuously denies.
Chelsea submitted a dossier of evidence to the FA containing signed witness statements from players, who allege Clattenburg used a term understood to have been interpreted as racist.
But Ferguson said: "I don't believe Mark Clattenburg would make any comments like that. I refuse to believe it. I think it is unthinkable in the modern climate. I just don't believe it - simple as that. There is no way a referee would stoop to that, I am convinced of that."
Reacting to Ferguson's claims, Chelsea boss Di Matteo said: "He likes to talk about other clubs. We tend not to, so I'll leave it up to him. It doesn't really affect me too much to be honest."
Ferguson was the latest big-name manager to weigh into the debate, with Arsenal boss Arsene Wenger also critical and Leeds' Neil Warnock - who was QPR manager when John Terry racially abused Anton Ferdinand - was even more scathing.
"Nothing new to us," Di Matteo said of the attacks.
"It's a free country where everybody has the freedom of speech. We take into consideration what other people say and use this for a motivational tool for ourselves."