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 readmit 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 in an interview with The Ecologist magazine for the American President.

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".

The mayor 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 recognise George Bush because he was not officially elected. So we are organising 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 comments will infuriate Downing Street at a time when it is examining ways of bringing Mr Livingstone, who was expelled for standing as an independent in the London mayoral elections of 2000, back into the Labour fold.

Tony Blair's spokesman said last night: "His views are for Ken Livingstone. They are a matter for him. The Prime Minister has made his views well-known in this respect and looks forward to meeting the President."