John Kerry, the Democratic presidential frontrunner, today tried to limit the damage to his campaign by denying a rumour that he had a "recent alleged infidelity" with one of his interns.

The Drudge Report, a gossip and news web site which reported yesterday that Mr Kerry had an "intern issue," claimed that a woman close to Senator Kerry recently left America at his behest.

When asked about the issue on Imus in the Morning, an MSNBC radio talk show, Senator Kerry said today: "There is nothing to report, nothing to talk about. There's nothing there. There's no story."

The woman in question is reported to be a 20-year-old former New York journalist who worked for the married father-of-two as an intern. The Drudge Report said last night that she had gone to Africa.

Democrats have accused their Republican rivals of dirty tricks. Many observers fear that this year's presidential campaign could be the dirtiest for years, citing the questions raised about President Bush's military service during the Vietnam War as further evidence.

Senator Kerry, from Massachusetts, is currently the runaway favourite to win the Democratic ticket to run against Mr Bush in November. He has won 12 of the 14 state primaries and caucuses held so far.

Key parts of the Drudge story had already been denied, before Mr Kerry's radio interview. The web site claimed that General Wesley Clark, one of Mr Kerry's Democrat rivals, had set the ball rolling by telling a dozen reporters that Mr Kerry had an "intern issue" which threatened to "implode" his campaign.

General Clark's campaign yesterday called the report "utter rubbish", and let it be known that he was expected to endorse Mr Kerry's campaign today, having dropped out of the race himself.

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 that the weekly magazine was as puzzled by the story as the rest of the world.

The mainstream media in the US has so far not reported on the claims.

Meanwhile another web site said that it had broken the Kerry story last week. A report dated February 6 on Watchblog said that there was a rumour that Time was going to report this week that Mr Kerry had had an affair with a 20-year-old woman - but that this was nothing more than a rumour at that stage.

Kerry, a decorated Vietnam veteran, is married to Theresa Heinz Kerry, the heiress to the Heinz food empire. Born on December 11, 1943 in Denver, Colorado he has two children from a previous marriage, and three stepchildren with his current wife.

Ms Heinz-Kerry, widow of Pennsylvania Senator John Heinz who was killed in an air crash in 1991, is known for her outspoken views.

She once told Elle magazine that she had warned her first husband on the subject of adultery: "If you ever get something, I'll maim you. I won't kill you. I'll maim you."