The Kallistem laboratory, a private research facility based in Lyon, claimed it would be able to carry out human clinical trials within two years.

If commercialised, the firm hopes to treat 50,000 patients a year, a market it that could be worth £1.7 billion.

But the findings have not been published, peer reviewed or independently verified – and British experts last night were treating the claims with scepticism.

Kallistem said it had managed to transform basic male fertility cells, called spermatogonia, into mature sperm in test tubes.