It was one of the internet’s most ambitious, radical and ultimately successful ideas.

Eight years ago Wikipedia, the free online encyclopaedia that allows anyone to write and edit articles, declared that it would provide access to “the sum of all human knowledge”. It soon became one of world’s most popular websites.

The site assumed that facts and information could be provided by all. Anyone was allowed to log on, write and change articles. Any subject — from Barack Obama’s election to characters in the Star Wars films — was considered worthy of inclusion. The pages have been updated and improved upon thousands of times and they are used more than 300 million times a month by everyone from primary school pupils to speechwriters — even if they should know better.

Surprisingly to sceptics, who have long warned that inaccuracies abound on the website and that they can come to be regarded as fact, the project seems to have proven the wisdom of crowds. A recent study suggested that its pieces were just as accurate as those in the Encyclopaedia Britannica.

Fears are increasing, though, that the Wikipedia project could be starting to stall. Research reveals that the volunteers who create the pages, check facts and adapt the site are abandoning the site in unprecedented numbers.

Every month tens of thousands of Wikipedia’s editors are going “dead” — no longer actively contributing and updating the site — without a similar number of new contributors taking their place.

Some argue that Wikipedia’s troubles represent a new phase for the internet. Maybe, as some believe, the website has become part of the establishment that it was supposed to change.

The research found that in the first three months of this year the English-language version of the site suffered a net loss of 49,000 contributors, compared with a loss of about 4,900 during the same period last year. Many experts believe that the trend could threaten Wikipedia’s future.

The research was conducted by Felipe Ortega at Libresoft, a research group at the Universidad Rey Juan Carlos in Madrid. He created a computer system that analysed the editing history of more than three million active Wikipedia contributors in ten different languages.

Mr Ortega told The Times: “If you don’t have enough people to take care of the project it could vanish quickly. We’re not in that situation yet. But eventually, if the negative trends follow, we could be in that situation.”

His study suggests that editors are leaving the site in record numbers. In the latest period for which there are figures, across March and April this year, it lost about 23,000 of its 100,000 English-language editors. Mr Ortega said that early indications suggested that the downward trend was continuing.

Wikipedia’s popularity among readers remains undiminished, however. The site’s founder, Jimmy Wales, started it in 2001 as an experiment and today it is the fifth most popular website in the world. Analysts believe that it receives about 325 million visits per month, a number that continues to rise.

The Wikimedia Foundation, the not-for-profit organisation that finances and runs the website, did not respond to questions about the new study. It has previously recognised the fall in the number of volunteers but argued that the encyclopaedia could remain useful.

Others were alarmed by the findings. “It’s definitely a worrying trend,” said Andrew Dalby, author of The World and Wikipedia: How We are Editing Reality and a regular editor of the site. “One question is, is there any new stuff to do on the site? When Wikipedia reaches 3 million articles, how many new articles can there be?”

Mr Dalby explained that contributors were becoming disenchanted with the process of adding to the site, which he said was increasingly difficult. “There is an increase of bureaucracy and rules,” he said. “Wikipedia grew because of the lack of rules. That has been forgotten. The rules are regarded as irritating and useless by many contributors.”

Wikipedia has been embarrassed by a number of cases in which the site was used to spread incorrect information, such as when the US Senator Edward Kennedy was declared dead prematurely. New rules were introduced to help stop these errors, but some writers claimed that they were being censored.

Last week the page dedicated to the French footballer Thierry Henry was locked after his handball during the World Cup play-off match with Ireland. Incensed fans left obscenities on the site, as well as a flood of criticism. Other rules have been introduced to reduce infighting and clashes between writers.

“There’s the frustration of realising that what you do will not stay there for ever,” Mr Dalby said. “Somebody else will come along and rewrite it, re-edit it, and maybe in your view, spoil it.”

Some experts argued that the pioneering ethos of early Wikipedians had faded. “They don’t feel the spirit of the first years,” Mr Ortega said. “The articles are very tightly controlled by others now, and that makes it hard to jump in and contribute.”