The BBC has warned Blue Peter presenter Konnie Huq about political bias after she took part in a press conference with London mayor Ken Livingstone.
The BBC said it had told Ms Huq's agent she should not attend
She appeared at an event to promote cycling despite the corporation telling her agent she should not take part.
Conservative London Assembly member Brian Coleman said the Labour mayor had turned the event into a "political rant", breaking BBC impartiality rules.
But Ms Huq's agent, Jonathan Shalit, said she had attended "with goodwill".
The BBC has apologised to Mr Coleman, adding that this was a "one-off incident".
Deputy director-general Mark Byford wrote to him, saying: "The BBC had turned this down on the grounds that it would be unsuitable for her and Blue Peter.
"It was felt that the BBC and the programme should not be linked with anything that might be construed as campaigning, and that this campaign potentially fell into that bracket."
He added that Ms Huq's contract had been re-written, giving the BBC an "absolute veto over what she can and can't do".
Mr Coleman said Mr Livingstone had used the event to accuse the Tories of pursuing "pro-car" policies.
He told The Times: "The launch became a political rant. It is unforgivable for the BBC to allow the Blue Peter name to be lent to a political event."
Mr Coleman later said he accepted the BBC's apologies.
Mr Shalit said: "It is totally not the BBC or Konnie's fault. It is quite correct the BBC said no to it but I wasn't aware of that.
"The BBC probably realised it was a political event when I didn't. All Konnie was asked to do was support a get-fit campaign. It was done with goodwill. The Tories have made this into a political event."
Mr Livingstone's spokesman denied there had been a "political rant", adding: "There was no party political element to Konnie Huq's involvement in the launch."
He added that, during the press conference, Mr Livingstone had "referred briefly" to the amount of money spent on cycling by Barnet Council, of which Mr Coleman is a member.
Mr Livingstone also mentioned that Mr Coleman had run up a £10,000 taxi bill, the spokesman said, adding that this had been a "moment of jokey banter".
He said: "It was certainly not in any way party political - no political parties were even mentioned.
"The Hovis London Freewheel is a new event to promote cycling and London and supporting this event cannot in any reasonable view be regarded as party political."