Ken Livingstone yesterday launched a savage attack on the UK's racial equality chief, saying he was so right-wing that "soon he'll be joining the BNP".
The London mayor said the criticism by Trevor Phillips, the head of the Commission for Racial Equality (CRE), of multiculturalism was "pandering to the right".
"I don't know where Trevor Phillips is going," Mr Livingtone told BBC London radio.
"I mean, I remember when we had the first mayoral election and he was running to be mayor - he denounced me as being a racist because I said to him 'would you like to be my deputy'?
'All of our customers are international and we need those transport links to be as efficient and effective as possible'
"He'd had a brief sort of black power fling - and ever since then he's gone so far over to the other side that I expect soon he'll be joining the BNP."
The cause of Mr Livingstone's outburst was a comment by Mr Phillips earlier this week that the Notting Hill carnival, was not a "triumph of multiculturalism" but a celebration of one community's culture.
"Although it's a fabulous party, carnival can hardly be said to represent the everyday culture of most of London's communities," he told the Royal Geographical Society.
He said it was actually avoided by many of London's other minority communities, who gave their spare time to their own celebrations, such as Chinese New Year.
Mr Phillips' comments came amid a new debate about the benefits of multiculturalism, which encourages different communities to live separately, as opposed to integration, where immigrants are encouraged to adopt their host country's language and customs.
Communities secretary Ruth Kelly last week raised questions about whether multiculturalism was working in Britain, or whether it was simply creating a collection of isolated groups of people who did not understand or trust each other.
Speaking on Wednesday, Mr Phillips said: "There are those who argue, of course that separateness doesn't really matter as long as we are equal. I disagree, for several reasons.
"First because separateness in and of itself tends to encourage inequality of treatment.Second because living separately means that different groups of people have their life experiences defined by their ethnicity rather than their ambitions."
However, yesterday Mr Livingstone insisted that Mr Phillips' criticism of multiculturalism was submitting to the racist tendencies of the far right.
"I think exactly what Trevor is doing is trying to move the race agenda away from a celebration of multiculturalism and pandering to the right, and I have to say it's absolutely disgraceful," he said.
A spokeswoman for the CRE said: "The CRE's and Trevor's views on multiculturalism have been well documented and are well supported. Obviously, the mayor is entitled to his opinions."