Alarm bells rang last month when newspapers in Massachusetts discovered that the staff of Congressman Marty Meehan had polished his biography by, 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 engaged in skulduggery - 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 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 encyclopedia employing full-time editors, Wikipedia accepts entries submitted by anyone. 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.

Robert McHenry, a former editor-in-chief of Encyclopaedia Britannica, said: "My thesis has been that, contrary to the Wikipedia idea of constant improvement, 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. 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.

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

In the very spirit of openness that provides such ammunition for the snipers, Wikipedia freely admits its weakness. In its own entry, the encyclopedia states that 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. It is available in more than 100 languages, and thousands of new entries are added every day.

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