Ken Livingstone, the Mayor of London, launched a stinging attack on President George Bush last night, denouncing him as the "greatest threat to life on this planet that we've most probably ever seen".

His provocatively timed comments, on the eve of Mr Bush's arrival in London tonight, threaten to create severe embarrassment for the Prime Minister. They also come with talks under way on whether to re-admit Mr Livingstone to the Labour Party before his five-year exile ends.

Although he made his many differences with the Government on a range of issues clear, he reserved his strongest comments for the American President in an interview with The Ecologist magazine.

The President's three-night trip, which will culminate on Friday with a visit to the Prime Minister's Sedgefield constituency, has sparked a flood of protests from those opposed to his foreign policy. But Mr Livingstone's outburst makes him one of the most high-profile and explicit of his critics.

Mr Livingstone recalled a visit at Easter to California, where he was denounced for an attack he had made on what he called "the most corrupt and racist American administration in over 80 years". He said: "Some US journalist came up to me and said: 'How can you say this about President Bush?' Well, I think what I said then was quite mild. I actually think that Bush is the greatest threat to life on this planet that we've most probably ever seen. The policies he is initiating will doom us to extinction."

Mr Livingstone, who is holding a "peace party" for anti-war groups in City Hall tomorrow, added: "I don't formally recognize George Bush because he was not officially elected. So we are organizing an alternative reception for everybody who is not George Bush."

He said he supported stronger links between European Union countries only because he wanted to see a powerful bloc emerge to rival the United States. "The American agenda is sweeping everything before it, and although it's not perfect, the EU is better on environmental issues. It's a less rapacious form of capitalism."

The Mayor said he had viewed Labour's 1997 election manifesto as a "load of old guff they'd come out with because they didn't want to upset the Daily Mail" that would rapidly be ditched. "I was amazed when it transpired that Blair had been serious," he said.

Accusing Mr Blair of suffering from a "background problem", he said: "There is nothing in his past that was radicalizing. He wasn't interested in all the great student activities, the radical campaigns.

"He did not get involved in politics until the 1970s, when the high point was passed. So you have someone of the summer of '68 generation who actually wasn't part of it."

On GM foods, he said: "If the Government ignores public opinion, then civil disobedience on this issue is legitimate, as long as it's not violent.

"But the most important thing that affects a government is not peaceful protest, but fear of the ballot box.

The Mayor's comments will infuriate Downing Street at a time when No 10 is examining ways of bringing Mr Livingstone, who was expelled from the Labour Party for standing as an independent in the London mayoral elections of 2000, back into the fold.

© 2003 Independent Digital (UK) Ltd

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