Boots, the high street chemist, is becoming the country's largest seller of quack medicine, according to Britain's leading scientific expert on alternative therapies.

Talking at the Hay literary festival today, Edzard Ernst, professor of complementary medicine at Exeter University, is to criticise the company for selling alternative medicines, in particular more than 50 homeopathic remedies, which are shown by clinical trials to be no more effective than sugar pills.

Boots, which has 1,500 stores across the UK, stocks 55 homeopathic therapies, 34 of which are sold under the company's own brand.

Ernst accuses the company of breaching ethical guidelines drawn up by the Royal Pharmaceutical Society of Great Britain, by failing to tell customers that its homeopathic medicines contain no active ingredients and are ineffective in clinical trials.

"The population at large trusts Boots more than any other pharmacy, but when you look behind the smokescreen, when it comes to alternative medicines, that trust is not justified. You can buy a lot of rubbish, with covert advertising stating things that are overtly wrong. People are spending their money on stuff that doesn't work," he said. "Boots seems to be fast becoming the biggest seller of quack remedies in UK high streets."

The ethical code for UK pharmacists states that those selling or supplying homeopathic and other complementary medicines must help patients make informed decisions by providing them with necessary and relevant information. It also calls on pharmacists to "uphold public trust and confidence" by acting with honesty and integrity.

"This can only mean that pharmacists should tell their customers that a homeopathic remedy they are about to buy doesn't contain a single molecule of whatever it says on the label, and that there's no clinical evidence that it works beyond a placebo effect," Ernst said.

Homeopathic remedies are highly diluted solutions that often contain no trace of their original ingredients. Instead, homeopaths claim the treatments work because "healing energy" is imprinted into the water when it is shaken. In 2006, Sir Martin Rees, president of the Royal Society, said the possibility of a medicine working in this way would "entail some fundamentally new scientific principle with amazingly broad ramifications".

"Very few people are aware that the underlying principles of homeopathy are totally scientifically implausible, and even fewer people are aware that the trials show it doesn't do anything," said Ernst.

The UK market for homeopathic medicine was estimated to be worth £38m in 2007 and is expected to reach £46m by 2012, according to a report by research firm Mintel. There are nearly 4,000 homeopathic practitioners in the UK and five NHS-funded homeopathic hospitals. Ernst and science writer Simon Singh will base their Hay lecture on their recent book, Trick or Treatment? Alternative Medicine on Trial, but will use the event to highlight what they view as unethical behaviour by the company.

Boots has previously funded a fellowship at Ernst's department at Exeter.

"People could end up harming themselves if they think one of these products will help them or their child, when in fact it's only a placebo. It's certainly a rip-off and they could be losing valuable time to see a doctor. This makes a fool out of evidence-based medicine, it is back to the dark ages," Ernst said.

Boots said that many of its customers believed that homeopathic remedies provide health benefits. "Homeopathy is recognised by the NHS, and many doctors and other health professionals see it as a useful option for medical treatment," said a spokesman.

"In addition, many of our customers also choose to use homeopathy. The Royal Pharmaceutical Society of Great Britain issues guidance to pharmacists on provision of homeopathy and homeopathic preparations to ensure that the sale and supply is appropriate."