This week, Simon Singh, one of Britain's best science writers, will decide whether to carry on playing a devilish version of Who Wants to be a Millionaire? He has already lost £100,000 defending his right to speak frankly. He could walk away. No one would think the worse of him if he did. Or he could go on and risk losing the full million by ensnaring himself in the rapacious world of an English judiciary that seems ever eager to bow to the demands of Saudi oil billionaires, Russian oligarchs and the friends of Saddam Hussein to censor critics and punish them with staggering damages and legal fees.

It seems no choice at all. Any friend Singh phoned would tell him to cut his losses and run. But if he were to turn to the audience, he would hear scientists all but screaming at him to go to the Court of Appeal and challenge a judgment that threatens the robust discussions open societies depend on. A national defence campaign is ready to roll on his command. At a preliminary support meeting, a cheering crowd acclaimed him as a free-speech champion.

In truth, he makes an unlikely warrior. Singh is a serious and amiable man, whose accounts of the solving of Fermat's last theorem and code breaking won high praise and provoked no controversy. Last year, he published Trick or Treatment? with Professor Edzard Ernst on the reliability of "alternative medicine", and devoted a chapter to the strange history of chiropractic treatments. One Daniel David Palmer invented the therapy in Davenport, Iowa, in 1895, when he convinced himself that he had cured a janitor's deafness by "racking" his back.

Inspired by this miracle, Palmer developed the theory that "95% of all diseases are caused by displaced vertebrae", rather than, say, the germs that so bothered conventional doctors of the time. Chiropractic therapy was a new religion, Palmer declared, and he was a successor to Christ, Muhammad and Martin Luther. At home, he practised vigorous racking on his children.

His son, Bartlett, described how he beat them with "straps until we carried welts, for which Father was often arrested and spent nights in jail". Bartlett bought the first car Davenport had seen and paid his father back by running him down on the day of the Palmer School of Chiropractic Homecoming Parade.

Palmer died of his injuries a few weeks later, but his ideas lived on. In 2008, the British Chiropractic Association (BCA) announced that its members could help treat children with colic, sleeping and feeding problems, frequent ear infections, asthma and prolonged crying. Writing in the Guardian, Singh said the claim was "bogus". Chiropractic treatments may help relieve back pain, but Professor Ernst had examined 70 trials and found no evidence that they could relieve other conditions.

Singh is hardly a lone sceptic. A few weeks ago, the Advertising Standards Authority upheld a complaint against a chiropractor who claimed he could treat children with colic and learning difficulties. Nevertheless, the BCA took Singh on and told me it had "numerous documents which demonstrate the efficacy of chiropractic" treatments.

Fair enough, you might think. Reputable medical authorities could test the evidence and decide whether the treatments work or not. Instead of arguing before the court of informed opinion, however, the BCA went to the libel courts and secured a ruling from Mr Justice Eady that made Singh's desire to test chiropractors' claims next to impossible. Because Singh used the word "bogus", the judge said he had to prove that chiropractors knew they were worthless but "dishonestly presented them to a trusting and, in some respects perhaps, vulnerable public".

The learned judge did not seem to understand that the worst thing about the deluded is that they sincerely believe every word they say. On Eady's logic, a writer who condemns as "bogus" a neo-Nazi's claim that a conspiracy of Jews controls American foreign policy could be sued successfully if lawyers jumped up and said neo-Nazis sincerely believed their conspiracy theories to be true.

The consequences of letting the libel law loose on scientific debate are horrendous. Science proceeds by peer review. A researcher's colleagues must submit his or her ideas to scrutiny without fear of the consequences. If they think they could lose their homes and savings in the libel courts, however, they will back off.

For alternative therapists are not the only ones answering their critics with lawyers. NMT, an American health giant, is suing a British doctor for questioning one of its treatments.

After the Singh ruling, the Sense About Science lobby group fears the commercial pressure to rush out new treatments will lead companies to quash doubters with writs in London courts and put public health at risk.

Watching recent libel cases has been like hearing rumours about parliamentary expenses. For years, I have wondered what it will take to turn a neglected scandal into a public outrage. After Eady ordered the censorship of a New York author's book on terrorism, which had not even been published in Britain, the US Congress began drafting a law which will guarantee that English libel judgments have no validity in America. The United Nations has condemned the judges' practice of welcoming rich libel tourists from across the world to their hospitable courts and urged Britain to allow free speech on matters of public interest.

In the Commons, MPs have railed against the absurdity of a legal system which forced a Danish newspaper to pay £100,000 for criticising the shady financial practices of the Icelandic bank Kaupthing, which duly collapsed six months later along with the rest of the Icelandic economy.

None of their despairing pleas has moved the government or persuaded the judiciary to reform itself. Maybe the Singh case will.

If he goes ahead with an appeal this week, bloggers, academics and the massed ranks of the scientific great and good are ready to join him. They have grasped what too many still fail to realise: the greatest threat to freedom of speech in Britain is not the state or the security services or the press barons, but a fusty and illiberal legal system, which has become a public menace.