That such a bizarre conspiracy theory broke out from internet chatrooms, grabbed the mind of a former minister and was splashed across the media reflects the growing popularity of conspiracy theories. Observers of the phenomenon, more pronounced in America than here, say that it is destabilising vulnerable individuals and undermining society.

Although Mr Meacher admitted that he got much of his information from websites, he made his claim in The Guardian. The newspaper evinced little scepticism and published letters from readers relieved that the truth had come out. David Aaronovitch, a Guardian columnist, expressed alarm that his newspaper had given credibility to such “rubbish”.

Mr Aaronovitch destroys Mr Meacher’s theory that there was a slow US reaction to the September 11 attacks. Mr Meacher claimed that not one fighter plane was scrambled to investigate until after the third plane had hit the Pentagon. But Mr Aaronovitch noted that six minutes after the notification of the first hijacking, fighters were ordered to be scrambled from Otis Base, in Massachusetts.

Mr Meacher also claimed that no serious effort had been made to catch Osama bin Laden, distorting the words of the US Chairman of the Joint Chiefs of Staff, General Richard Myers, who told journalists: “Finding one person, as we’ve talked about before, is a very difficult prospect, but we will keep trying.”

According to David Alexander, author of Conspiracies and Cover-Ups — What the Government Isn’t Telling You, “There’s been a tremendous increase in conspiracy theories about September 11.”

It’s not just that. In 1998 Rich Buhler, an American radio show host, set up Truthorfiction.com to track and prove or disprove hoaxes, urban myths and conspiracy theories. His organisation now gets 1,000 such stories a week.

Mr Buhler said: “The internet has proven a valuable tool for conspiracy theorists — they exchange ideas and fuel the stories. They have a much larger canvas than before.There’s an impression that whatever is written is more reliable than what is said: when it is written down, like it is on the internet, it comes packaged as truth.”

Yet there are other reasons why conspiracy theories are gaining currency — in particular the complexity of our rapidly changing world. “A conspiracy theory becomes more compelling when reality makes less sense, when life is beset by problems, when the established order suddenly changes — even something as simple as losing a job. If people can’t absorb what’s going on, conspiracy theories help us to make sense,” Mr Alexander said.

A limited understanding of the world makes conspiracy theories seem more plausible. “If people just know a little more, they would know the thing is false,” he said.

People are becoming further removed from seats of political and industrial power. Patrick Leman, a psychologist at Royal Holloway College, London, who has been studying why conspiracy theories are so appealing, said: “Conspiracy theories feed into a feeling of disconnection with government. People don’t like gaps in their accounts; they have a need to believe them. They invent fantastical things that protect them from the real world.”

In one experiment, he showed people footage of a fictional president who was shot at, and provided fictional newspaper articles. People were more likely to believe that there was a conspiracy behind the shooting if he was killed than if he was uninjured. He concluded: “People think that a big event must have a big cause, but often things are caused by cock-up or accident, not conspiracy.”

Thousands of people die in Europe every year in car accidents resulting from fast driving and too much alcohol. But when Diana, Princess of Wales died, many could not accept that such an important event could have such a simple cause. Many believed that she was assassinated by the secret services to stop her marrying a Muslim.

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