Virgin Radio boss Chris Evans is to be investigated by the Radio Authority after his on-air backing for London mayoral candidate Ken Livingstone.

His station, Virgin Radio, could be fined or have the terms of its licence altered if the authority decides its guidelines on impartiality have been breached.

Virgin has already been warned by the watchdog, which will be studying tapes of Tuesday morning's breakfast show to investigate the matter further.

On Monday, Evans announced he was donating £100,000 to Mr Livingstone's campaign, prompting the indepdendent candidate's rival - the Labour candidate Frank Dobson - to say: "I am somewhat relieved really because my mum always told me to steer clear of redheads."

In response, ginger-haired Evans upped his donation to £200,000, telling listeners on Tuesday morning: "What Frank Dobson said is very offensive to people with red hair.

"Kids growing up with red hair could be given a really tough time, especially if they have glasses."

Immediately after the programme, a Virgin spokesman distanced the station from from his comments, saying his donation was "entirely personal".

"The views expressed are his own and are in no way representative of the radio station."

Impartiality rules

The Radio Authority issued guidelines earlier this month to stations reminding them of their obligation to be impartial in the run-up to the poll on 4 May.

The Broadcasting Act requires "due impartiality" on matters of political controversy, while non-news programmes must avoid giving undue prominence to one candidate at the expense of others.

A spokeswoman said: "We are investigating and it is up to members of the authority to adjudicate on this matter when they meet in May."

The Authority has the power to fine Virgin, shorten or even revoke its licence, but this has never occurred to date. The largest fine it has issued is £50,000.

Virgin's national AM licence runs out in 2001, while its London FM licence lasts until 2003. However, it has an automatic eight-year extension to its national franchise as it is part of the new digital radio services.