The Drudge Report, an online gossip and news site, claimed a woman had recently fled the US, prompted by the Massachusetts senator. It alleged that Mr Kerry was fighting off claims of “recent alleged infidelity”.

Even though there was no evidence for the claims, and key parts were denied, the report prompted feverish speculation among Washington insiders and highlighted the influence of unconventional internet sites in modern US politics.

The website claimed that it had nine million hits within hours of the report being posted, and it presented the Kerry campaign with the most awkward of dilemmas: should it ignore the claims or issue a denial that itself would make headlines.

Mr Kerry, resting after the rigours of the primary campaign, spent the day inside his elegant Washington home. His campaign staff, anxious to avoid at all costs adding any fuel to the fire, refrained from commenting.

But in the cyberspace age, the claims appeared around the world, leaving millions wondering whether this election, in a country split right down the middle, was going to live up to its billing as one of the dirtiest ever.

Key parts of the allegation were immediately challenged. The Drudge account claimed that Wesley Clark, the retired Nato commander who bowed out of the Democratic presidential race this week, had set the ball rolling by telling reporters that Mr Kerry had an “intern issue” which threatened his campaign.

Interns are young unpaid staffers who work for senior American politicians.

General Clark’s campaign called the report “utter rubbish”, and let it be known that he is expected to endorse Mr Kerry’s campaign today.

Drudge, notorious for running gossip spurned by the mainstream media, has a mixed record. It broke the story of President Clinton’s relationship with Monica Lewinsky, but had to apologise to one of Mr Clinton’s top aides for alleging that he beat his wife.

Drudge claimed that half a dozen news outlets were investigating the allegations against Mr Kerry, but most of them denied doing so last night.

Leonard Downie, executive editor of The Washington Post, mentioned by Drudge, said: “This is the first we’ve heard that we’re working on a story that we’re not working on.”

A reporter with Time magazine, also cited by Drudge, said the weekly magazine was as puzzled by the story as the rest of the world.

Sources within the Clark campaign blamed Republican “dirty tricks”, and gave warning that it was a taste of things to come. “They are just throwing mud,” he said.

Mr Kerry had already planned to take two days off the campaign trail to rest his sore throat and recover from the energies of his twin primary victories in Virginia and Tennessee on Tuesday.

Mr Kerry’s campaign offices in Massachusetts and Wisconsin referred all queries about the allegations to the campaign headquarters in Washington.

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