The full extent of the campaign of intimidation, attacks and death threats made against scientists by activists who claim researchers are suppressing the real cause of chronic fatigue syndrome is revealed today by the Observer. According to the police, the militants are now considered to be as dangerous and uncompromising as animal rights extremists.

One researcher told the Observer that a woman protester who had turned up at one of his lectures was found to be carrying a knife. Another scientist had to abandon a collaboration with American doctors after being told she risked being shot, while another was punched in the street. All said they had received death threats and vitriolic abuse.

In addition, activists – who attack scientists who suggest the syndrome has any kind of psychological association – have bombarded researchers with freedom of information requests, made rounds of complaints to university ethical committees about scientists' behaviour, and sent letters falsely alleging that individual scientists are in the pay of drug and insurance companies.

"I published a study which these extremists did not like and was subjected to a staggering volley of horrible abuse," said Professor Myra McClure, head of infectious diseases at Imperial College London. "One man wrote he was having pleasure imagining that he was watching me drown. He sent that every day for months."

Chronic fatigue syndrome – also known as myalgic encephalomyelitis (ME) – is common and debilitating. A recent BMJ (formerly the British Medical Journal) feature suggested that as many as one in 250 people in the UK suffers from it. Patients are sometimes unable to move and become bedridden, occasionally having to be fed through a tube. For more than 20 years, scientists have struggled to find the cause, with some pointing to physiological reasons, in particular viral infections, while others have argued that psychological problems are involved.

It is the latter group that has become the subject of extremists' attacks. The antagonists hate any suggestion of a psychological component and insist it is due to external causes, in particular viruses. In the case of McClure, her "crime" was to publish a paper indicating that early studies linking the syndrome to the virus XMRV were wrong and the result of laboratory contamination. So furious was the reaction that she had to withdraw from a US collaboration because she was warned she might be shot.

A similar hate campaign was triggered by a study published in the Lancet earlier this year. It suggested that a psychological technique known as cognitive behavioural therapy could help some sufferers. This produced furious attacks on the scientists involved, including Michael Sharpe, professor of psychological medicine at Oxford University. He had already been stalked by one woman who was subsequently found to be carrying a knife at one of his lectures.

"The tragedy is that this tiny group of activists are driving young scientists from working in the field," said Sharpe. "In the end, these campaigns are only going to harm patients."

This point was backed by Fiona Fox, director of the Science Media Centre. "Using threats and intimidation to prevent scientists pursuing specific avenues of research or speaking out is damaging not just science. It harms society," she said.

None of the scientists contacted by the Observer believed chronic fatigue syndrome was purely psychological. All thought external causes were involved. "There is an element that is heritable," said Dr Esther Crawley, a consultant paediatrician at Bristol University. "We also know that in children it is often triggered by a virus infection, while in adults it is associated with social deprivation. Stress and adversity is involved. To call this yuppie flu – as people have done – is a complete misnomer."

Crawley has spent years trying to unravel the causes, but her refusal to accept that the condition is a result only of organic external factors has resulted in her being deluged with hate mail from extremists. "You evil bastards … time is running out for you so you have [sic] better start denouncing your flawed inhumane therapy and pray to God for forgiveness," said one.

"To those who are responsible for preventing us sick ME sufferers from getting the help we need ... you will all pay," stated another. "It is depressing to receive emails like that, but I make sure that it does not get me down," said Crawley. "I do check packages that are sent to my office, however."

Many of the extremists' claims are bizarre, said Professor Simon Wessely, of the Institute of Psychiatry at King's College London. "They say I am in league with pharmaceutical companies in order to suppress data that shows a link between viruses and the syndrome. But why on earth would drug companies do that? If they could link the condition to a virus they would be well on the way to developing lucrative treatments and vaccines. It is crazy."

Wessely has installed speed dial phones and panic buttons at the police's request and has his mail X-rayed. He gave up his research on chronic fatigue syndrome several years ago, though he still treats patients. "I have moved my research interests to studies of Gulf war syndrome and other conditions linked to war zones," he said. "That has taken me to Iraq and Afghanistan where quite frankly I feel a lot safer – and I don't mean that as a joke."