Evans counts the cost of supporting Ken: £100,000 (plus a £75,000 fine)

Virgin Radio has been fined £75,000 - the largest penalty imposed by the Radio Authority - over Chris Evans's on-air support of Ken Livingstone.

Mr Evans, who hosts the station's breakfast show, twice announced his backing for Mr Livingstone in the run-up to the London mayoral elections.

Richard Hooper, the authority chairman, said the incidents were a "flagrant breach" of rules on political impartiality which govern broadcasters. The fine is the station's fifth since it began broadcasting in 1993.

Mr Hooper said that the broadcast breached impartiality provisions of the 1990 Broadcasting Act as well as the authority's code. These had previously been brought to the attention of all stations because of the impending election campaign.

The issue arose after Mr Evans revealed he had donated £100,000 to Mr Livingstone's campaign. Frank Dobson, the Labour candidate, then made a remark about red-haired people which prompted Mr Evans to announce he would double his donation.

The authority said: "The station had not only failed to ensure its presenters were fully aware of the rules on due impartiality but allowed a second breach to be aired." The fine would have been higher if the station had not taken the steps it did to "make amends" for the breach the following day.

Mr Hooper said: "Political impartiality is a cornerstone of British broadcasting ... This broadcast by Virgin was a flagrant breach of the long-standing rules surrounding political impartiality ...

"The breach also demonstrated a critical failure of compliance by the licensee and we are determined that this shall not be repeated."

Guidance from the authority issued on 10 March said: "Non-news programmes must avoid giving undue prominence to any single candidate at the expense of others."

Virgin Radio accepted the ruling. In a statement it said: "As soon as the breach was identified the station took steps to ensure no further comments were made and immediately apologised." John Pearson, chief executive of Virgin Radio, said: "Chris Evans ... was drawn into making comments during an extended election campaign. For this we have apologised."

Mr Evans pledged the £100,000 to Mr Livingstone in a newspaper interview which sparked Mr Dobson's comment: "My old mum told me to steer clear of redheads". The broadcaster then upped the figure to £200,000.