Boris backs Barack as 'America's only hope'

The Mayor said only the Democratic candidate favourite to win the contest on 4 November could restore America's reputation in the world's eyes.

And he said Senator Obama would help bring an end to race-based politics by proving it was possible to reach the top irrespective of colour or background.

However, Mr Johnson's intervention just days before America goes to the polls breaks the convention that politicians do not become involved in other countries' elections.

His remarks also threaten to put him at odds with Tory leader David Cameron, who has met both candidates, and who praised Senator Obama's Republican rival John McCain earlier this year.

Tories traditionally support the Republicans and Senator McCain spoke at the Tory annual conference in Bournemouth two years ago.

The Evening Standard revealed this summer that Mr Johnson backed Senator Obama, saying his victory would be a "fantastic" boost for black people around the world.

Mr Johnson wrote in today's Daily Telegraph: "Obama deserves to win because he seems talented, compassionate, and because he offers the hope of rejuvenating the greatest country on earth in the eyes of the rest of us.

"Unlike his opponent, he visibly incarnates change and hope, at a time when America desperately needs both."

He added: "Then there is the final, additional reason, the glaring reason, and that is race.

"If Obama wins, the US will have at last come a huge and maybe decisive step closer to achieving the dream of Martin Luther King, of a land where people are judged not on the colour of their skin but by the content of their character.

"Then black people the world over will be able to see how a gifted man has been able to smash through the ultimate glass ceiling. He will have established that being black is as relevant to your ability to do a hard job as being left-handed or ginger-haired.

"We could even see the beginning of the end of race-based politics, with all the grievance-culture and special interest groups and political correctness that come with it."

Mr Johnson also attacked President George Bush, accusing him of having pulled off an "astonishing double whammy" of wrecking both democracy and capitalism through the Iraq invasion and the banking system's collapse.

"To have rocked one of those pillars may be regarded as a misfortune. To have damaged the reputation of both, at home and abroad, is a pretty stunning achievement for an American president," he said.

While he praised Senator McCain as "brave and principled", he said he did not offer Americans any hope of repairing those fundamental US ideals.