If your encyclopaedia told you David Beckham was an 18th Century Chinese goalkeeper, that the Duchess of Cornwall carries the title Her Royal Un-Lowness or that Robbie Williams earns his living by eating pet hamsters in pubs "in and around Stoke", you might consider seeking a second opinion.

Despite its breakneck journey toward global internet phenomenon, such questions of accuracy have dogged the online encyclopaedia Wikipedia (see link below) since its launch five years ago.

Fresh concerns about the ease with which Wikipedia's entries can be manipulated have been raised after it was announced that US politicians had been altering their profiles to make them more flattering.

Alarm bells rang last month when newspapers in Massachusetts discovered that the Congressman Marty Meehan's staff had polished his biography, for instance deleting his long-abandoned promise to serve only four terms and praising his "fiscally-responsible" voting record.

Detective work by Wikipedia found that other offices on Capitol Hill had been engaging in skullduggery - not all of them with flattering results, such as the false reference to Oklahoma's Tom Coburn being voted "most annoying senator".

Wikipedia said it was reversing changes to several of the politicians' entries, and by so doing, added to the list of controversies about its veracity.

One of the best known happened in December when the prominent US journalist John Seigenthaler complained that his Wikipedia entry implicated him in the assassination of US President John Kennedy.

The decision of a member of the public, Brian Chase, to insert the claim "as a joke" to fool a colleague exposed the flaw at the heart of Wikipedia - its openness.

Unlike a conventional encyclopaedia employing full-time editors, anyone can submit an entry to Wikipedia and have it published to millions of internet users.

And anyone can edit existing entries, rendering them inaccurate or offensive.

Wikipedia believes that this constant editing of an entry will lead to its ultimate perfection.

Others see it as a process ripe for misinformation and they do not hold back in their disparagement.

"My thesis has been that, contrary to the Wikipedia idea of constant improvement, that it is far more likely that on average bad articles will get better, good ones will get worse, and the mass tend to the mediocre," warns Robert McHenry, former editor-in-chief of Encyclopaedia Britannica.

"There are no standards of writing or research... At any given time one can easily find articles that are so badly written as to be unintelligible, while others are quite good. Some that are rife with error, while others seem authoritative."

McHenry sums up: "The problem for the ordinary user is that it is often not possible to distinguish the one sort from the other."

In the spirit of the very openness that provides such ammunition for the snipers, Wikipedia freely admits its weakness.

In its own entry, the encyclopaedia states openly there has been "controversy over its reliability" and lists its perceived problems as "systematic bias, difficulty of fact checking, use of dubious sources, exposure to vandals, privacy concerns, quality concerns, fanatics and special interests, and censorship."

But it also points to its strengths, principal among them the sheer, extraordinary mass of information - some 3.3 million entries - available to the public totally free.

Wikipedia is available in more than 100 languages, each separate from each other, and thousands of new entries are added every day.

Wikipedia is one of the biggest experiments into the web's democracy, communality and usefulness, and arguably its most successful exponent of those virtues.

Check on the Battle of Waterloo and find a sober, succinct account of Napoleon's downfall, explore the culinary use of chrysanthemums or delve into the early life of the Babyshambles' singer Pete Doherty.

Even the non-league club Burton Albion is honoured with a 702-word history, including its current manager, the name and capacity of its stadium and its home and away strip.

Such global dominance was built from unpromising beginnings.

Wikipedia (wiki wiki means 'quick' in Hawaiian) was founded in January 2001 as a sideline to the Numedia encyclopaedia being written by experts for an American company, Bomis, whose main interest was internet pornography.

In 2003, Bomis handed the burgeoning encyclopaedia to a not-for-profit organisation headed by one of its executives, the Wikipedia co-founder Jimmy Wales.

The Wikipedia Foundation is funded by public donations and has just three employees, a lead software developer, Wales's assistant and an intern.

But there is an army of between 600 and 1,000 unpaid administrators, developers, stewards and bureaucrats, who maintain the site.

A bigger pool of 13,000 regular contributors edits at least five entries a month each.

Such checking leads to a daily battle of wits with the cyber-wreckers who insert erroneous, ludicrous and offensive material into entries.

How frequently entries get messed about with depends on the controversy of their subjects.

This week the entry Muslim is being attacked dozens of times a day following the row about cartoons of Mohammed with angry denunciations of suicide bombing and claims of hypocrisy.

Prime Minister Tony Blair's entry is a favourite for distortion with new statements casting aspersions on his integrity.

One concluded a list of his various jobs such as First Lord of the Treasury with the line "and most of all George Bush's Bitch Boy".

Despite the constant battle to keep clean entries about controversial subjects, studies attest to Wikipedia's accuracy.

In December the respected science magazine Nature reported that Wikipedia was about as reliable on science subjects as the Encyclopaedia Britannia.

Nature found on average that Wikipedia had four inaccuracies per entry compared with three for its more conventional rival.

The magazine noted: "Only eight serious errors, such as misinterpretations of important concepts, were detected in the pairs of articles reviewed, four from each encyclopaedia."

In an evaluation of 66 articles, a German computer magazine, c't, rated Wikipedia 3.6 out of 5 for accuracy, beating two other online encyclopaedias, including Microsoft Encarta, which scored 3.1.

Wikipedia does not yet have the sheen of respectability for academics, even though it is increasingly used by people from all walks of life.

Wikipedia's co-founder Jimmy Wales, president of the Wikimedia Foundation, based in Florida, acknowledges that the encyclopaedia is imperfect.

"We regard everything as work in progress, and are pleased with the overall quality, but know there is a lot of work to be done so try not to be too complacent," he says.

But he does believe that Wikipedia is better than traditional reference sources, at least in terms of current affairs and technology - a specialism for many of its contributors.

"We pose a serious challenge to traditional encyclopaedias," Wales contends.

"When it comes to technology and current affairs, we are better, but in the humanities, a biography of the 1943 Nobel Prize Winner is likely to be shorter than in a traditional encyclopaedia, though reasonably accurate - we need more participation from volunteers who have humanities specialisms."

He insists: "We do a great job on global, breaking news stories, synthesising reports from a wide variety of sources."

Despite its undoubted strengths, the very idea of an encyclopaedia written by the public, no matter how expert, still unsettles librarians and academics.

Antony Beevor, the historian, says: "What I have found with Wikipedia's entries is that there is a lack of satisfaction, not so much through inaccuracy but there are a lot of vague statements which you cannot really disprove but which you don't think are necessarily helpful."