Ken Livingstone has accused Britain's race chief Trevor Phillips of being so right-wing that "he'll soon be joining the BNP".
The London mayor's comments, made on live radio, prompted immediate calls for an apology.
His BNP jibe came after Mr Phillips, the chair of the Commission for Racial Equality (CRE), said the Notting Hill Carnival was not a triumph of multiculturalism, but mere "domestic tourism".
In a speech on Wednesday, Mr Phillips also said the current wave of immigration was changing the face of Britain. The cost could no longer be ignored, he warned.
The two politicians first fell out in 2000, during the race to lead London.
"I don't know where Trevor is going," said Mr Livingstone on BBC London's Vanessa Feltz show. "When we had the first mayoral election Trevor Phillips denounced me as a racist because I said to him, Would you like to be my deputy?'
"Ever since then he's gone so far to the other side that I expect he'll soon be joining the BNP."
He added: "I think Trevor is trying to move the race agenda away from the celebration of multiculturalism and pandering to the right, and I have to say I think it's absolutely disgraceful."
The mayor also said Mr Phillips had an "absolutely disgraceful record" as race chief. "When he was appointed to run the CRE, it did an awful lot of work taking up genuine cases. What he did was turn it into a vast press department and wound down all the legal work."
In response, the CRE said in a statement: "Trevor's views on multiculturalism have been well documented and are well supported. Obviously, the mayor is entitled to his opinions."
The commission tried to resolve race issues without "costly legal action", it added. Most organisations responded to their warnings.
"If anything, our press team has shrunk in size and now consists of two press officers, a head of communications and a director. Our legal team has grown and consists of 39 officers in England and Wales."
The Tories also defended Mr Phillips.
"Trevor Phillips is doing a sterling job in a very sensitive position, and for the mayor to suggest that Mr Phillips joins the BNP is an absolute disgrace," said Brian Coleman, the Conservative chair of the London Assembly.
"I demand an immediate apology," he added. "We should expect our elected officials to show some respect, and not resort to childish outbursts."
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