The mayor wants a 9ft statue of Mandela
A statue of Nelson Mandela would not be suitable for Trafalgar Square, a panel of art experts has said.
The Mandela Statue Fund, which was launched by London mayor Ken Livingstone in March, hopes to raise £400,000 to create and erect the piece.
The bronze statue by artist Ian Walters would be placed on the new pedestrianised north terrace of the central London square.
But Westminster City Council's public art advisory committee has said it would be out of place because it is too big and in too prominent a location.
Mr Livingstone has called for the council's planning committee, which meets in June, to ignore the recommendation.
A spokeswoman for the Westminster Council said it had not rejected the statue idea "out of hand" because no application for it has been made as yet.
Given this is a world square, in a world city, a world class statesman is absolutely the right person to have here
London mayor Ken Livingstone
She added that it could choose to reject the art committee's advice.
But Mr Livingstone said: "I can't think of any other politician in the world as respected as Nelson Mandela.
"And given this is a world square, in a world city, a world class statesman is absolutely the right person to have here."
Westminster Council has told BBC London it did not object in principle to the statue.
Director of planning Carl Powell said: "It undermines the real debate here, which is about how do we create a great asset and also commemorate a great man to London.
"Let's judge it on its merits and not on the basis of the ethnic background of individuals."
The fund was initiated by the late South African newspaper editor and anti-apartheid activist Donald Woods.
It is now being led by his wife Wendy and Lord Attenborough.
The South African embassy is on one side of Trafalgar Square and was the scene of many demonstrations by anti-apartheid campaigners in the 1970s and 1980s.