The world's most popular online encyclopaedia has been transformed into a political battleground in the United States, with politicians' aides accused of "vandalising" entries on opposition figures.

Wikipedia, which is written by an army of volunteers and is open to anybody to edit, said US Congress staff were responsible for altering several of its biographical entries. The site launched an inquiry after staff for Marty Meehan, a Democratic representative, admitted having "polished" his biography.

The partisan editors were traced through their computers' unique IP addresses - which were tracked back to Senate machines. On several occasions, Wikipedia had been used as a platform for negative propaganda, the site said.

In one case, the Republican Senator, Tom Coburn was falsely accused of being voted "most annoying senator".

However, most of the edits have been aimed at burnishing politicians' CVs - and cutting out embarrassing material.

According to a report on Wikipedia's own news service, staff in the offices of Senator Joe Biden removed a paragraph concerning an alleged plagiarism scandal, as well as changing the section regarding a possible 2008 Presidential campaign "to read very positively".

On another page, "references, citations, and descriptions" of Conrad Burns' use of the term "ragheads" were removed, Wikipedia said. The staffers of Senator Tom Harkin removed a paragraph relating to the politician's having allegedly falsely claimed to have flown combat missions over North Vietnam.

The site also said that staff working for Senator Norm Coleman removed references to his voting record during his first year in Congress, where he had agreed with President Bush "98 per cent of the time".

Erich Mische, Senator Coleman's chief of staff, admitted to AP, the news agency, that his office had made changes to the site, including removing details of the politician's voting record, "that probably should have stayed in there".

Wikipedia, which started in 2001, has become a major online force, a factor that explains Washington's recently discovered interest. It now records around 2.5 billion page impressions a month. According to Mr Jimmy Wales, the site's founder, its traffic volumes are doubling every four months.

The site has made it clear that despite its trademark claim to be the reference work that "anyone can edit", it considers the recent bout of partisan edits to be improper. Mr Wales said: "If they're trying to edit in such a way to change the public record, that's a problem."

Mr Wales himself ran into controversy last year when he admitted editing his own Wikipedia entry. He said such behaviour, though tempting, "was in bad taste".

The site's reputation for reliability was also tarnished when John Seigenthaler, the founding editorial director of USA Today, was falsely linked with the assassination of President John F Kennedy by a Wikipedia article. Mr Seigenthaler described the site as a haven for "volunteer vandals with poison-pen intellects".

However, according to a recent study by Nature, the scientific journal, Wikipedia is actually no more unreliable than the venerable Encyclopaedia Britannica – the standard to which the website aspires.

Nevertheless, Mr Wales has insisted that users of the service should be aware of its limitations.

He recently told Times Online: "If what you’re after is 'who won the World Cup in 1986', it’s going to be fine – no problem.

"If you want to know something a little more esoteric, or something that’s going to be controversial, you should probably use a second reference – at least."

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