A Chinese factory supplying fake medicinal drugs has been exposed for providing dangerously powerful Viagra to British chemists.

An Insight investigation has identified a factory in China supplying Viagra that is three times the maximum strength of the genuine product. It is life-threatening to men with heart conditions.

Samples of the factory’s Viagra, a drug for sexual dysfunction, were tested by Pfizer, its official manufacturer, and found to contain talcum powder as well as “overdose” levels of sildenafil, the drug’s active ingredient.

“Just one of those tablets would give you an overdose,” said a Pfizer spokeswoman. “The effects in men with existing diseases, such as cardiovascular conditions, may be unpredictable and potentially serious.” Other patients would be at greater risk of side effects such as headaches and dizziness The pills were supplied by Gabriel Zhang, a 32-year-old businessman, who showed an undercover reporter around his factory in a remote village of Henan province in eastern central China. Zhang boasted: “If you’ve been into a chemist in the UK and bought some Viagra, there’s a chance it might have come from my factory.”

Like many counterfeiters, he is expanding his business from sex drugs to more lucrative life-saving medicines. The trade is causing concern among regulators as thousands of packs of drugs have already reached chemists in the UK.

Zhang was secretly taped by a reporter offering a monthly supply of 600,000 fake packs of life-saving prescription drugs. He agreed to manufacture large amounts of fake Casodex, a treatment for prostate cancer, at 7p a tablet when it wholesales in the UK at £4.57.

He sent the Viagra and Cialis, another sex drug made by Eli Lilly, to show how good his copies looked. Details of Zhang’s factory are being passed to the Chinese authorities.

Among faked drugs sold to patients with NHS prescriptions are Casodex, Plavix, a blood thinner, and Zyprexa, a schizophrenia treatment. This weekend Boots, the high street chemist, disclosed that in June it had written to 8,000 patients who had purchased the batches. The chemists was one of several that unwittingly bought the drugs through wholesalers.

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