Ken Livingstone has pulled out of a BBC mayoral election debate after it emerged a British National Party candidate would be taking part.
The former London mayor said it was a "point of principle" that he would not share a platform with the far-right party.
Labour's candidate also told BNP voters he did not want their second preference votes when they get to the ballot box in next month's vote.
The debate, due to be hosted by BBC London 94.9 on Monday, is the latest in a series of head-to-head mayoral broadcasts but most have featured just the top three or four candidates.
Mr Livingstone said: "I have long held to the belief in no platform for the far right. The far right want to destroy our democracy and stand for the elimination of our basic rights.
"They cannot be treated as a legitimate part of politics.
"I am withdrawing from Monday's mayoral debate on BBC London 94.9 on the grounds that I am not prepared to share a platform with the British National Party.
"We have been in negotiation about a debate with the main candidates for BBC London 94.9 - but only now have we been informed that the BNP had been invited to take part in this debate, and this has been confirmed to us today.
"I will not share a platform with the BNP and it is a point of principle to me that I never will do."
BNP candidate Carlos Cortiglia told political blogger Iain Dale he planned to give Mr Livingstone his second preference vote.
Mr Livingstone said: "That the BNP candidate may have called for a second preference for me is clearly designed to disorganise progressive opinion and divide opposition to them.
"Not only don't I want their second preference but I will not share a platform with them. I urge everyone to use their vote on May 3 to vote to keep the BNP out."
A BBC spokesman said: "We would like to hear from all the candidates in a studio debate but this is a big logistical challenge and we appreciate it might not be possible.
"It is up to individuals to decide whether to take part.
"The BNP has demonstrated evidence of electoral support - over a series of different types of elections and by winning representation in the London Assembly in 2008 - for which we will give them proportionate coverage".