Pfizer lost its UK patent rights to its erectile dysfunction drug Viagra over the weekend, leading to an influx of cheaper generics into the market.

In the UK last year the NHS wrote 2.3 million prescriptions for Viagra (sildenafil), at a cost of £40 million.

But Pfizer lost its UK patent on Friday, meaning the price of the drug will be reduced from £10 to around £1, cutting the bill for the drug by 90 per cent. Generic firms such as Teva along with 19 other companies have already said they are launching their own forms of sildenafil.

But to help combat generic competition, Pfizer said it would launch its own cheaper brand, named ‘Sildenafil Pfizer’, to avoid being outdone by competitors.

The company said men would still need a prescription to purchase the drug on Viagra.com, but would not need to see a pharmacist.

Jonathan Emms, the company’s UK managing director, said Viagra would continue to sell because most people asked for the medicine by name. He told the Times newspaper: “The brand is very well established and well known among patients.”

The British Generic Manufacturers Association said, however, that competition would be ‘fierce’, adding: “This is potentially a substantial market and we would expect many companies to launch generic versions when the patent expires.”

The impotence treatment has been one of the world’s bestselling drugs, with sales reaching $2 billion in 2012, with around half of this coming from the US.

But the now famous diamond blue pill has been a target for illegal versions of the drug, and Pfizer has been battling for years to combat illicit copies sold on the internet.

The firm recently announced that it would sell its drug online in May via its Viagra.com website, although this is only available for US men. It hopes that this will mean fewer men will use illegal sites to buy the drug, and also forgo the perceived embarrassment of visiting a doctor for a prescription.

Dr Tom Brett, medical director of Lloydspharmacy Online Doctor, said: “We hope that the black market through illegal websites will shrink. Hundreds of websites have been shut down and 68,000 illegal doses were seized last year - although it's impossible to know the extent of the problem.”

The drug was first launched 15 year’ ago as the first treatment of its kind for ED, and was discovered at Pfizer’s former European R&D headquarters in Sandwich, Kent.

Since then a number of other drugs for the condition have become available, including Lilly’s Cialis and GlaxoSmithKline and Bayer’s Levitra, and more recently Vivus’ drug Stendra.

Pfizer still has another seven years of patent protection in the US, where it made nearly $1 billion last year.