Conservative leader David Cameron said: “I think this is a very big moment for the Conservative Party, but I don’t want anyone to think that we would deserve to win an election just on the back of a failing government.”

Bookmakers slashed the odds on a Conservative victory in the next parliamentary election to the shortest price for a decade.

John Curtice, politics professor at Scotland’s Strathclyde University, said the Conservatives fared better than expected while Labour had done even worse than forecast.

Brown enjoyed a brief honeymoon with voters after he took over from Blair. But he has since been beset by party mutinies, economic turmoil and industrial unrest.

Attention now turns to London where two political mavericks are battling for the job of mayor in the closest election since the office was created eight years ago.

Victory for Conservative candidate Boris Johnson in the race would be a major boost for Cameron. A win for Labour incumbent Ken Livingstone would provide some relief for Brown.

Turnout was expected to have been high, which pollsters said could favour Livingstone. The result is due later on Friday.

“Ken Livingstone stands between Gordon Brown and . . . a disaster,” Curtice said.