Wikipedia has been forced to abandon its policy of allowing anyone to edit its pages.

An army of 20,000 unpaid 'expert editors' will be drafted in to check all changes to articles on living people before the pages go online.

The move is a response to the hijacking of the site by those with political or personal motives.

Wikipedia founder Jimmy Wales says that the change in the system is just a test

Tory and Labour politicians, as well as 'web vandals', have falsified entries to discredit their enemies.

Wikipedia was set up eight years ago as a free encyclopedia built on the work of volunteers.

All contributors had the power to edit, improve and update the content and it has become one of the top ten internet sites with more than 13million entries.

But well-publicised hoaxes have forced the Wikimedia Foundation, the nonprofit Californian body that runs the site, to curb its freewheeling ethos.

They hope the switch to volunteer editors will curb malicious tampering and reduce the risk of lawsuits. Wikipedia tried to clamp down on the problem in 2005 by banning anonymous users from creating entries.

Experts said the latest change was much more significant and 'crosses a psychological Rubicon'.

The system of 'flagged revisions' will compromise the founding principle that everyone has an equal right to edit any Wikipedia page.

But Michael Snow, who is the chairman of the Wikimedia board, said it was no longer acceptable 'to throw things at the wall and see what sticks'.

Jimmy Wales, one of the site's founders, said: 'We have really become part of the infrastructure of how people get information. There is a serious responsibility.'

With millions of changes made to entries every month, it is thought that 20,000 editors will be needed.

Modified pages go live only with their approval.

Wikipedia is the first reference point for many web inquiries - often because its pages head the search results on Google and Yahoo.

More than 30million visits have been made to the Michael Jackson page since his death on June 25.

'Wikipedia now has the ability to alter the world that it attempts to document,' said New York University professor Joseph Reagle.

A limited number of popular or controversial pages are already protected, including those for singer Britney Spears and U.S. president Barack Obama.

Wikipedia's credibility took a dent when it emerged in 2005 that a biography of American journalist John Seigenthaler, once an assistant to US Attorney General Robert Kennedy, had been altered to accuse him of involvement in the assassinations of both his boss and JFK.

In one notorious case David Cameron’s aides altered the page on the artist Titian to score a point over Gordon Brown.

And in 2007 it emerged one of its main contributors had faked his qualifications.

Ryan Jordan, who had edited more than 20,000 pages of information, had claimed to be a professor of theology but was exposed following a magazine article as a 24-year-old college dropout from Kentucky.

Last year, the New York Times worked with Wikipedia to restrict information about the kidnapping of a correspondent in Afghanistan.