Slimming pills, antidepressants, and drugs for skin complaints, hair loss and stroke prevention are being sold online alongside treatments for cancer. The drugs are posted to buyers who have not been seen by doctors.
On Monday, regulators will attempt a crackdown on what they describe as "an explosion" in the websites, some of which sell counterfeit medication as well as drugs which are banned in Britain.
The Medicines and Healthcare Products Regulatory Agency (MHRA) will send out 600,000 leaflets warning patients of the dangers of buying prescription drugs online, and targeting nine products which have previously been counterfeited for sale in the UK.
The watchdog will also ask doctors' regulators to tighten the rules allowing British medics to prescribe drugs to patients they have never seen.
Our investigation revealed websites selling dangerous drugs with few or no checks.
The law states that patients buying drugs over the internet must first undergo a full online "consultation".
Several websites based in this country remained within the law by insisting on a free "consultation" with a doctor registered with the General Medical Council – essentially, a basic online quiz – before supplying prescription drugs.
Yet in some cases they failed to ask for crucial details necessary to identify risks to the patient.
Other websites, operated from abroad, offered prescription drugs without any checks at all, and sold diet pills banned in this country because of links to depression and suicide.
According to unpublished figures compiled by the MHRA and obtained by this newspaper, 500 cases have been referred to UK regulators because sites were believed to be operating illegally, including sites selling counterfeit drugs.
In more than 400 of the cases the websites were found to be hosted abroad, beyond the reach of British prosecutors.
Side effects for the anti-balding treatment Propecia include migraines, depression and erectile dysfunction.
But in our investigation, healthexpress.co.uk, a London-based website, supplied 84 tablets after asking the customer to fill in a questionnaire which did not ask whether there was any history of such complaints.
When we approached the company later for an explanation, it did not respond.
Another British website, doctorchemist.co.uk, sold the smoking cessation aid Champix without asking if there was any history of depression, vomiting or insomnia, all of which can be caused by the drug.
A spokesman for doctorchemist.co.uk said he was "very concerned" by the findings and asked for full details of the purchase we made in order to launch a full investigation.
He added: "The website relies on the honesty of the patient. It was not set up to allow people without prescriptions to get hold of prescription drugs."
A reporter posing as an overweight 25-year-old woman, and providing false doctor's details, successfully used the linked websites hwprescriptionservice.com and assetchemist.co.uk to buy 84 capsules of the slimming drug Xenical at a cost of £59 – £10 for the prescription and £49 for the drug.
Xenical is dangerous for women who are breastfeeding, but the online registration form, which asked whether the applicant was pregnant, failed to ask whether she was breastfeeding.
The completed prescription was delivered by email two hours after the reporter completed the online "consultation" form.
Asked later about the lack of checks, David Ojei, a representative of hwprescriptionservice.com, admitted: "That is very odd." No-one else at the company was available for further comment.
Other websites in this country sold prescriptions for as little as £10, which patients could take to pharmacies to buy an array of drugs.
Our investigation also found sites hosted in Asia, Russia and eastern Europe offering a range of drugs with serious side effects at the click of a button with no checks whatsoever.
Many offered pills known to carry serious risks, such as the slimming drug Accomplia, which was taken off the shelves of British pharmacies last October after regulators ruled that its risks, including that of serious psychiatric disorders, outweighed any benefits.
The MHRA's head of intelligence said the probe, which follows the disclosure of figures showing one in four family doctors has treated patients who had been harmed by drugs which were bought online, exposed the need for the rules to be tightened.
Nemo Ahmed said: "The data, and several deaths linked to online drugs in this country and abroad, means this is an area we are increasingly concerned about.
We would like the General Medical Council to make the guidance on this tighter and more enforceable, and we will be meeting with them to discuss this".
The investigation also uncovered a recent change in the law which has increased the risks to patients, by allowing doctors in dozens of foreign countries to sign off prescriptions for customers buying from UK websites.
Laws which came into operation in November, following a European Commission ruling, mean UK-based websites which prescribe drugs can contract out their work to doctors in 31 countries including Slovakia, Poland and Estonia.
Peter Walsh, chief executive of Action for Victims of Medical Accidents, a charity, called the change "extremely worrying".
He warned that British customers who believed that websites in this country were safer than those abroad could unwittingly receive prescriptions signed off by doctors whose English was poor and whose training did not meet UK standards.
In 2007, a north London GP who prescribed large amounts of dangerous addictive drugs over the internet was suspended from the medical register for nine months.
Dr Julian Eden, who founded the first British website to prescribe drugs online, admitted prescribing beta blockers to a suicidal 16-year-old boy despite his history of self-harming and psychiatric care.
The teenager overdosed on the drugs after obtaining a repeat prescription, without being given any advice to see his own GP.
Last summer, Shabbir Hussain, from Cricklewood in north London, was sentenced to four years in prison after a raid on his home found a garden shed packed full of counterfeit medicines including slimming pills and Viagra, which Hussain intended to sell via websites.
The Department of Health ruled out an outright ban on internet pharmacies based in the UK, saying it would "reduce choice and convenience" for the public, but advised anyone thinking of buying from such a site to consult their own GP first.
The GMC, which regulates doctors, said current guidance made clear that doctors who prescribe online must gather sufficient information to prescribe safely, but said it would work with the MHRA on the issues raised.
