In Britain last year, tens of thousands of men bought Viagra online. (The worldwide market for the drug is worth £1.1 billion.) And while the internet may save the buyer money - prescription Viagra costs between £5 and £7 per pill, compared with as little as £2 on the net - the price to human health could prove more costly.

Viagra is just one example of the counterfeit drug production that is becoming a "worldwide problem", according to Naeem Ahmed, the head of intelligence at the Medicines and Healthcare products Regulatory Agency (MHRA). Three weeks ago the American group Pfizer, one of the world's leading pharmaceutical companies, recalled 120,000 packets of its cholesterol-lowering drug Lipitor 20mg after 73 fake packets were discovered.

However, last week, the American company uncovered another batch of the fake drug, The Telegraph can reveal.

While the counterfeit Lipitor is of "no immediate threat to the public", according to Lisa O'Neil, a Pfizer spokesman, because it was intercepted well before reaching pharmacy shelves, the situation is indicative of the escalating problem in this country.

It marks the fourth time since the beginning of 2004 that a substantial cache of fake drugs has been discovered in Britain. Lipitor, which is sold for £24 per pack of 28, brings in £12 billion annually for its manufacturer, which makes counterfeiting a highly lucrative trade.

Last year, counterfeit batches of Cialis, also used to treat impotence, and Reductil, used to treat obesity, were discovered. The World Health Organisation has estimated that about eight to 10 per cent of medicines available globally are counterfeit. Drug forgery has long been recognised as a problem for the developing world.

In Nigeria more than 60 per cent of drugs are bogus, according to Graham Satchwell, who wrote a report on counterfeit medicines for Interpol last year. In China 100,000 people die every year as a result of fake drugs.

Mr Satchwell said: "What's to say it won't happen here? I am convinced there have been fatalities in the UK. To put a number on it is very difficult indeed. But where there are clear cases of fake anti-cholesterol drugs, which contain only half or none of the active ingredient, deaths must have gone unnoticed. 'Show us the bodies,' says the Government. 'You're not looking,' I say."

Mr Satchwell recommends stricter drug testing and greater security in the distribution chain.

What takes place in the drug forgery business has remained largely concealed from public knowledge.

John Theriault, the vice-president of global security for Pfizer, has given this newspaper details of a raid in Colombia in 2004, at a site where fake Ponstan (an anti-inflammatory drug) and Terramycin (an antibiotic) were manufactured. An informant working for another pharmaceutical company led Colombian authorities to the plant in Bogota, where 800,000 tablets were seized.

The tablets were being produced in extremely unhygienic conditions, and were made of boric acid, a pesticide that can cause gastric problems, or death if ingested in sufficient quantities. Brick dust was used as a binding agent.

"The Ponstan was dyed yellow, its authentic colour, with paint used to mark the highway," said Mr Theriault. "Floor wax was also used to give the pills a sheen."

An undercover investigation carried out by Pfizer in Thailand in 2001 has also come to light. Posing as drug buyers, Pfizer's security agents purchased Viagra on the internet at heavily discounted prices then, claiming to want to distribute the drugs, arranged to meet the Turkish distributor in Bangkok.

Raids on two locations followed, leading to the seizure of 80,000 counterfeit Viagra tablets, packaging and manufacturing equipment, in addition to the confiscation of two million Valium tablets.

In places such as Egypt, where incriminating photographs were taken by authorities, counterfeit drugs are produced in extremely insanitary conditions.

"The products are not safe, they are not sterile. These are not professionals," Mr Theriault said. "Products are made out of anything people can get their hands on."

Miss O'Neil said: "There are cases where people are making cement in the day and fake drugs at night."

In Britain, four people have been arrested in the past month for selling counterfeit drugs over the internet.

On August 12, Keith Morgan, from Milton Keynes, was sentenced to 100 hours of community service for selling unlicensed medicines on the net.

A package addressed to Morgan was intercepted in the post. It contained 2,680 Kamagra tablets, another pill used to treat impotence. Morgan admitted that he had a contact in India who sent the tablets to him. Morgan was charging £25 for four tablets.

Mr Ahmed said: "We are not talking about counterfeit bags and jeans, but people's lives. No supply chain is impenetrable, but we are working hard to control the situation."