Ken Livingstone has released a broadside against Commission for Racial Equality (CRE) chief Trevor Phillips, saying he is failing minorities.
The London mayor described Mr Phillips as a "failure" and said plans to bring together all agencies dealing with discrimination in one place were "rubbish".
"If Trevor Phillips wasn't black, he couldn't have done what he's done," Mr Livingstone said in an interview with LBC Radio.
"If a white man had been put into the CRE with the job of winding it down, there would have been uproar and they wouldn't have got away with it."
A spokeswoman from the CRE described the mayor's comments as "unfortunate".
The new Commission for Equality and Human Rights (CEHR), which will be headed by Mr Phillips, is designed to act as a central point for advice and guidance on all equality and human rights issues in the UK Britain from Autumn 2007.
It will combine the roles of the CRE, the Disability Rights Commission and the Equal Opportunities Commission.
The London mayor also disagreed with these plans.
"I was opposed to winding [the CRE] up. This idea that we're going to have disabled people, lesbians and gays, women and blacks all in one human rights commission, I think, is rubbish," he said.
It is not the first time Mr Livingstone has criticised Mr Phillips.
In September the London mayor described the CRE chief as so right-wing that "soon he'll be joining the BNP".
"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'?
"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 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 Livingstone insisted 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."