The tumbling cost of Viagra is set to give 1.5 million men in the UK a much-needed boost.

The price of the anti-impotence pill has fallen from £21 for four down to £1.45 after its patent expired last year - with the result that it can now be prescribed more readily on the NHS.

A spokesman for the Department of Health told The Sun: "Erectile dysfunction is a common and distressing condition.

"Now that this treatment is much cheaper we can make it more widely available on the NHS."

Health bosses expect to see a "significant increase in demand".

It is thought the number of people who get the pill will double or even triple from the 750,000 figure today.

Impotence of varying degrees is believed to be affect half of all men aged between 40 and 70 - an estimated 2.3million in the UK.

In 2012, when pharmaceutical giant Pfizer still had the monopoly on Viagra, the NHS spent  £40million on the drug.

The pill was patented in 1996 but this expired in June.

The rise of the internet saw a surge in men buying Viagra online, worrying doctors who warned some versions were fakes.

However, since June Pfizer’s rivals have been free to manufacture legitimate versions to sell more cheaply.