Scientists and specialists who question medical treatments and alternative therapies are being gagged by firms using Britain’s draconian libel laws.

Researchers who have raised concerns about osteopathy, heart implants and homeopathy have been forced to withdraw or water down scientific papers and websites for fear of being sued.

One expert was threatened with legal action even though he was merely repeating a critical ruling made by the UK’s advertising watchdog.

Claimants have been accused of using the laws to suppress scientific debate. Defending a libel case is difficult because the onus is on the defendant to prove their innocence and even success can leave individuals tens of thousands of pounds out of pocket.

Last night Sir Ken Macdonald, the former director of public prosecutions, called for legislative reform.

“The idea that we are becoming an international haven for people to attack scientists is something that we should not be proud of,” he said. It was “shameful” that a US drug company could sue a doctor in Britain for criticising a treatment while it would not countenance taking similar legal action in America.

“The way we have allowed London to develop as the international libel centre of the world is not really at peace with a society that believes freedom of expression is a human right.”

Andy Lewis, who debunks spurious medical claims on his website, had to withdraw an article about a form of osteopathy after being threatened with litigation.

Lewis, from Oxfordshire, had reported a ruling by the Advertising Standards Authority that banned a company called Optimum Health Centres from claiming that one of its practitioners, Robert Delgado, was able to treat serious ailments such as migraines and whiplash by adjusting patients’ spines. The company had also claimed that Delgado’s technique could boost the immune system by 200%.

When lawyers for Optimum claimed that Lewis’s article was defamatory, he decided not to risk a costly legal battle.

He said: “I was a bit surprised because I didn’t think it was controversial — it was about some official findings. Rather than going to the expense of responding with a lawyer, I took it down from the website.”

Another victim of the libel laws is Dr Peter Wilmshurst, a consultant cardiologist at Shrewsbury hospital who is being sued by an American company, NMT Medical, after he questioned the effectiveness of a new heart implant device.

Wilmshurst raised his criticism at an American conference and his comments were posted on a US website, but he is being pursued at the High Court in London because “a few” cardiologists read the article in Britain, according to his lawyer.

A mediation hearing is scheduled for next month. NMT Medical, of Boston, Massachusetts, denies engaging in libel tourism and using the law to stifle debate.

Last year the International Journal of Speech, Language and the Law removed from its website a study into lie detector technology used by councils across Britain to catch benefit cheats.

The authors of the paper, Anders Eriksson and Francisco Lacerda, found “absolutely no basis” for claims that the technology — layered voice analysis — could identify when telephone callers were not telling the truth. They described its Israeli manufacturers, Nemesysco, as charlatans.

The article was swiftly pulled when Amir Liberman, the founder of Nemesysco, threatened to sue.

“People said those comments were not libellous but we couldn’t predict the outcome of the case,” said Professor Peter French, one of the journal’s editors. “The costs would have been considerable and it was for us to prove that Liberman was a charlatan.”

Liberman, however, said he had acted fairly. “It would have damaged my reputation if it [the article] had stayed there,” he said. “Saying this is censorship is ridiculous.”

Some scientific journals are so fearful of litigation that they refuse to publish potentially controversial papers.

Professor Edzard Ernst, director of the complementary medicine group at Exeter University, said the Journal of Medical Ethics had rejected a paper in which he had claimed homeopaths were violating their own ethical code.

“It was galling because they had accepted it and then their lawyer advised them not to publish on the grounds that it could risk a libel suit,” he said.

The article was eventually published in another journal, but Ernst claimed that many other papers he writes now have to be “toned down”.

Simon Singh, the author of Fermat’s Last Theorem who is being sued by the British Chiropractic Association for describing some of their treatments as “bogus”, said: “Libel laws are suppressing scientific debate ... because the cost of defending a case is so great.”

The Commons media select committee is expected to call for changes to libel laws in a report next month.

Its members are likely to recommend placing a cap on costs, ensuring that cases can be heard in the English courts only if the material they allude to was meant to be read by a sizeable audience in this country, and distinguishing between material published on the internet and that in newspapers and magazines.

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