John McCain’s campaign has become embroiled in the revelation that the teenage daughter of Sarah Palin, his newly announced running mate, is five months pregnant.

Bristol Palin, 17, one of the Alaskan governor’s five children, is going to keep the child and marry the father, Mrs Palin and her husband Todd announced in a statement released by the McCain campaign.

It came in the wake of swirling internet rumours that Mrs Palin had faked her own recent pregnancy, as Democrats began to sense that there might be more to emerge about the past of the Republican vice-presidential pick.

Senior aides to Mr McCain said the nominee knew of the pregnancy when he selected Mrs Palin last week to be his running mate on the Republican ticket. Yet with journalists and Democratic researchers poring over her public and private life, new details emerged today making clear that the vetting process by the McCain campaign into his vice-presidential choice was rushed and cursory.

Aides to Mr McCain said the decision was taken to reveal Bristol Palin’s pregnancy to rebut what one called the “mud-slinging and lies” circulating on liberal blog sites, which suggested that Trig, Mrs Palin’s fifth child born in April with Down’s Syndrome, actually belonged to her daughter, and that the candidate had faked her own pregnancy to cover up for her child.

The rumours began when Mrs Palin announced for the first time on March 6 that she was already seven months pregnant, a fact that few had guessed by her appearance. In the preceding months her daughter Bristol had been absent from school for sustained periods, after being diagnosed with Mononucleosis. Mrs Palin gave birth to Trig - knowing from tests that he had Down’s Syndrome - on April 18. Mrs Palin, a social conservative who opposes abortion , returned to work three days later.

“The despicable rumours that have been spread by liberal blogs, some even with Barack Obama’s name in them, is a real anchor around the Democratic ticket, pulling them down in the mud in a way that certainly juxtaposes themselves against their ’campaign of change’,” a senior McCain aide said.

Referring to the accusation, Mr Obama said: "I am offended by that statement. There is no evidence that this involves us." He vowed to sack any member of his staff found to be spreading the rumour.

In their statement - released as Mrs Palin spent the day cloistered near the Republican convention in Minnesota working on Wednesday's acceptance speech - the Palins said: “Our beautiful daughter Bristol came to us with news that as parents we knew would make her grow up faster than we had ever planned."

They added: “Bristol and the young man she will marry are going to realise very quickly the difficulties of raising a child, which is why they will have the love and support of our entire family. We ask the media, respect our daughter’s privacy as has always been the tradition of children of candidates.”

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