Mayor's laughing all the way to Banksy - thanks to mystery bidder

The work, Sketch For Essex Road, was bought over the phone by a dealer to keep the new owner's identity secret.

An assistant from the gallery that bid on behalf of the client refused to reveal any details. But the name will come out eventually because all donations have to be declared to the Electoral Commission.

Last night's art auction at the Aquarium Gallery in Farringdon raised more than £230,000 for the Mayor's fighting fund. Thirty painters and sculptors including Marc Quinn, Antony Gormley, Mona Hatoum, and Sex Pistols designer Jamie Reid donated works.

It gave him a much-needed boost after months of pressure, including the departure of former race and policing adviser Lee Jasper after a series of allegations of misconduct.

The new cash means Mr Livingstone is well on his way to catching up with Conservative rival Boris Johnson, who has raised more than £350,000 from wealthy donors towards his £1.2 million target. The Mayor is also understood to have raised at least £100,000 from the unions so far.

After the auction the Mayor thanked the artists who had donated their works. He told the Evening Standard: "I was sitting there totting up as we went along and thought I'd make £100,000, because we got about £30,000 from everything else and we expected £75,000 for the Banksy.

"I felt completely light-headed when it went for almost £200,000 - and I've only had two glasses of wine.

"I suppose the artists are aware of what is happening. They recognise that if Boris gets in there will be barbarism, so they're doing everything they can to help."

The Banksy painting, an acrylic and oil on canvas, features two children with hands on hearts pledging allegiance as a third child hoists a fluttering Tesco bag on a flagpole.

It was the last work to go under the hammer and bidding was intense, leaping from £30,000 to £100,000 in one go and then soaring further. Banksy's spokeswoman refused to comment on the donation.

Many of the bidders at the sale appeared to be dealers buying work on behalf of clients or speculating on paintings for their own galleries.

However, there were several well known Labour figures in the crowd including Deputy Mayor Nicky Gavron and millionaire Labour MP Geoffrey Robinson, who paid £5,000 for a large oil painting called Heartbeat by Jo Wonder.

Ms Gavron appeared to be the purchaser of a carbon sketch by Turner prize winner Antony Gormley, which went for £6,800.

Mr Livingstone, a huge newt fan, paid £140 for Lizard Lounge, a signed poster by cartoonist Ralph Steadman.

Photographers Peter Kennard and Cat Picton Phillipps donated Photo-Op, a print of a grinning Tony Blair snapping himself on his mobile phone while Iraq burns behind him. It went for £750. No Third Runway, their print of Gordon Brown walking out of Downing Street onto a miniature Heathrow, fetched £600.