Ken Livingstone has blocked the appointment of nine white men because they are “unrepresentative” of London.
The Mayor used his powers to promote equality to stop them serving on the London fire authority.
He refused to confirm all but one of the Tory and Liberal Democrat nominations to the body that sets the brigade’s budget and policies.
The Mayor’s opponents are expected to seek a High Court injunction to overturn his ruling saying that it has caused “unnecessary and damaging uncertainty”.
Only five Labour members of the 17-strong body are likely to turn up for Thursday’s annual meeting.
Mr Livingstone rejected the nominations of five Assembly members and four borough councillors because they “failed to tackle the under-representation of women and black, Asian and ethnic minority Londoners”. The authority is made up of nine Assembly members and eight councillors according to each party’s political strength across the capital.
The Mayor said he had taken the step after the Tories nominated six white men and one white woman, while the Lib-Dems selected three white men.
He said: “It is unacceptable that when there are 1,861 councillors in London, of which 555 are women and 293 from black, Asian and ethnic minority groups, all seven Conservative nominees are white and include only one woman, and all three Liberal Democrat nominees are white men.
“To fulfil my duty to promote genuine equality I have decided not to accept these unrepresentative nominees and to ask those who are making these nominations to ensure that they reflect London as it really is.”
The Mayor approved all five Labour nominations, as well as one Tory woman and the sole Green and One London party representatives.
Merrick Cockell, Tory chairman of London Councils, said: “It is disappointing that the Mayor has rejected two pensioners, an openly gay councillor and a councillor under 30 as being ‘unrepresentative’ of London’s diverse communities.
“The fact remains these councillors were democratically elected by London’s diverse communities.”
Mike Tuffrey, Lib-Dem leader on the authority, said: “It is not for the Mayor to dictate to the other parties who they appoint.”