On 10 June a revealing letter from the McTimoney Chiropractic Association was published on Andy Lewis's excellent website, The Quackometer. The strongly worded letter from the MCA advises all its members to take down their websites immediately or risk prosecution. The letter refers to "a witch hunt against chiropractors" with campaigners targeting "any claims for treatment that cannot be substantiated with chiropractic research".

The use of the phrase "witch hunt" brings to mind visions of the Salem witch trials or the worst excesses of the McCarthy era, with innocent people being unjustly persecuted by those in power. Challenging unsubstantiated treatment claims does not seem to me to qualify as a witch hunt.

The letter goes on to advise members to "REMOVE all the blue MCA patient information leaflets, or any patient information leaflets of your own that state you treat whiplash, colic or other childhood problems in your clinic" and, "If you use business cards or other stationery using the 'doctor' title and it does not clearly state that you are a doctor of chiropractic or that you are not a registered medical practitioner, STOP USING THEM immediately."

They were also warned to "Be wary of 'mystery shopper' phone calls and 'drop ins' to your practice, especially if they start asking about your care of children, or whiplash, or your evidence base for practice."

The letter concludes: "Finally, we strongly suggest you do NOT discuss this with others, especially patients. Firstly it would not be ethical to burden patients with this, though if they ask we hope you now have information with which you can respond." It is reassuring to see that the MCA takes its ethical responsibilities so seriously.

Just in case any of its members had not got the message, the MCA letter states: "IF YOU DO NOT FOLLOW THIS ADVICE, YOU MAY BE AT RISK FROM PROSECUTION."

What caused the MCA to react with such panic? As most readers will already know, the lack of good clinical evidence relating to the use of chiropractic for treating a range of disorders with no direct link to problems of the spine has come under the spotlight as never before following the decision of the British Chiropractic Association to sue science writer Simon Singh.

In an article in the Guardian last year he criticised the BCA for claiming that its members could use spinal manipulation to treat children with colic, ear infections, asthma, sleeping and feeding conditions, and prolonged crying. Singh described the treatments as "bogus" and based on insufficient evidence, and criticised the BCA for "happily promoting" them. At a preliminary hearing last month to decide the meaning of the article, a judge ruled that Singh had implied that the BCA was being consciously dishonest.

Could this explain the MCA's apocalyptic letter to its members? Apparently so. When the Guardian approached the association to check the authenticity of the leaked letter, it responded with a statement:

Following the High Court decision and in what one can only speculate was a spirit of retribution, a number of Dr Singh's supporters decided to launch, in their own words, a "blitzkrieg" against the chiropractic profession. This has centred on trawling the websites of chiropractors and one individual, Alan Henness, has made complaints against over 500 individual chiropractors to the Statutory Regulator for chiropractors, the General Chiropractic Council (GCC).

It continues:

For a chiropractor, having a complaint made against you to the GCC is a very serious matter. The process of having a complaint investigated by the GCC is a very stressful, protracted and ultimately expensive process for the chiropractor, however minor or serious the misdemeanour, regardless of the eventual outcome ... As soon as the MCA became aware of the actions of the 'skeptics', as they like to call themselves, we advised our members to withdraw their web sites as a precautionary measure in light of what was considered to be a vexatious campaign against the profession.

And in conclusion:

The MCA has nothing to hide – and it is our belief that our members have not intentionally breached any rules regarding the content of their websites. The MCA was not alone in advising such precautions; indeed at least two other chiropractic associations have given similar advice to their members.

The BCA's use of the perverse English libel laws in an attempt to silence Singh has caused outrage and concern in equal measure among scientists, journalists, and indeed anyone who values free speech and honest debate. Detailed coverage of the case can be found at Jack of Kent's superb blog.

An interesting development took place on 20 May when the Advertising Standards Authority (ASA) published its adjudication on whether chiropractors Dr Carl Irwin and Associates "could substantiate the implied claim that their therapies could successfully treat some of the conditions mentioned, in particular IBS, colic and learning difficulties". The relevant part of the adjudication reads as follows:

We considered that, whilst some of the studies indicated that further research was worth pursuing, in particular in relation to the chiropractic relief of colic, we had not seen robust clinical evidence to support the claim that chiropractic could treat IBS, colic and learning difficulties.

On these points the ad breached CAP Code clauses 3.1 (Substantiation), 7.1 (Truthfulness) and 50.1 (Health and Beauty Products and Therapies).

The ASA instructed that the offending advertisement must not appear again and that the practice must not refer to the treatment of IBS (irritable bowel syndrome), colic and learning difficulties in future. Furthermore, the chiropractors gave assurances that they would not refer to themselves as "doctors" in any future advertising to avoid giving the misleading impression to members of the public that they held general medical qualifications.

Some people were so angered by the BCA's legal assault on Singh that they decided to follow up the ASA ruling. Among them was Simon Perry, organiser of the Leicester branch of Skeptics in the Pub. Singh's own account can be found on Perry's Adventures in Nonsense blog.

Essentially, Perry downloaded from the BCA website the details and websites of 174 chiropractic practices that either claimed to treat colic or else implied that chiropractic was an effective treatment for this condition. Using their postcodes, he then found their local trading standards office using the Trading Standards Institute website. Having checked the content of each chiropractic website, he then mailed 84 letters of complaint to trading standards offices, referring to the ASA adjudication and saying he believed the practices in question to be in breach of the unfair commercial practice regulations and other consumer protection legislation.

He has posted the latest results on his Adventures in Nonsense blog. It appears that out of the 84 chiropractors he reported to trading standards with individual letters, 14 have now removed the word "colic" from all pages on their web sites. Out of 55 individual chiropractors that Perry reported to the General Chiropractic Council, 13 have removed the word from all of their web pages.

Perry kindly agreed to perform a similar mail-merge on my behalf. My letters, around 80 of them, simply pointed out that as far as I was aware there is no convincing scientific evidence that chiropractic is an effective treatment for colic.

At the time of writing, I have received some 46 replies. The vast majority of these are simply acknowledging receipt of my complaint and telling me that it will be investigated. The others display a wide range of decisions regarding my concerns.

Trading Standards Service of East Sussex County Council informed me that they had been in touch with Lushington Chiropractic to advise them of the ASA's adjudication and to ask them to remove any reference to the conditions mentioned. Interestingly, their letter to me also states that "The General Chiropractic Council have recently contacted them regarding this, and so they have already taken steps to have the statements removed from their website and literature as soon as possible."

A similar response was received from the Borough of Poole Environmental and Consumer Protection Services with regard to Amethyst Chiropractic Clinic. A few responses informed me that the practices in question appeared to have already removed any references to the treatment of colic (possibly as a result of the MCA letter?).

Other responses simply reported that the trading standards officer in question had decided not to take any action against the chiropractic practice. Some, such as the environmental protection department of Sefton Council, proclaimed that my complaint had been investigated and, in its opinion, the Back for Your Future Family Chiropractic Clinic was not contravening any of the legislation that it enforces. No justification was provided for this verdict.

Four letters were particularly worrying. The trading standards officers for Haringey Council, the City of Edinburgh Council, and the Highland Council all claimed that they could only take action against the practices if they could "prove beyond all reasonable doubt" that the claim that chiropractic can treat colic is false or misleading. There is no obligation on the part of the trader, I was informed, to establish anything.

Mark McGinty of the Highland Council informed me that "For your information, it would appear that the evidence available is inconclusive … " and Simon Keegan, the trading standards inspector employed by the Northern Ireland Trading Standards Service, told me that "there is no conclusive scientific evidence stating that chiropractic does not offer effective treatment for infantile colic."

As it happens, to the best of my knowledge, there is no conclusive scientific evidence that chewing my toe-nail clippings is not an effective cure for Aids, but I would rather hope that trading standards services would take some action against me should I ever decide to set up a business based upon this claim.

The most commonly cited paper in support of the treatment of infantile colic by chiropractic is by Klougart, Nilsson, and Jacobsen (1989). They showed that 94% of a group of 316 infants suffering from colic improved when treated with chiropractic. The appropriate response to this statistic is "So what?" As no control group was included in the study, we simply cannot know if the results might have been even better if no spinal manipulation had been employed.

A subsequent study by Olafsdottir, Forshei, Fluge, and Markestad (2001) did include a placebo control group and concluded that "Chiropractic spinal manipulation is no more effective than placebo in the treatment of infantile colic." This raises the question of what exactly would constitute proof "beyond all reasonable doubt", to the satisfaction of certain trading standards officers, that chiropractic is ineffective as a treatment for colic?

Overall, though, the effect of this campaign appears to have been very positive from the perspective of those who value science, free speech and open debate. The BCA has shot itself in the foot by taking the heavy-handed approach it did with respect to Simon Singh's article. By doing so, it has brought the lack of good evidence for many of chiropractors' claims right out into the bright light of day.

If you care about science and free speech, join the 12,000 people who have already signed up to support Simon Singh and keep the libel laws out of science at the Sense About Science website.

Chris French is a professor of psychology at Goldsmiths in London where he heads the Anomalistic Psychology Research Unit. He edits the [UK] Skeptic magazine