The anti-impotence drug Viagra is being bought over the internet and then sold illegally at almost twice the price in pubs and clubs in London, a BBC investigation has revealed.

There are hundreds of online sites where Viagra can be bought.

Although most require some form of online consultation, this is often cursory.

It is impossible to tell how much Viagra - and other prescription drugs - are coming into the country in this way.

There is also growing evidence that those who peddle illegal drugs in clubs are starting to deal in Viagra. Combined with the illegal drug Ecstasy it is well known on the club scene as Sexstasy.

Drug worker Mark McPherson, from Turning Point, said: "We are aware anecdotally of combinations of Viagra and Ecstasy being used on the club scene."

One dealer told BBC's Today Programme: "I go on the internet, type in the world Viagra, millions of sites pop up.

"You have got loads of options. It is well cheap, you pick up the tabs for �8 each, you can buy a maximum of 90 tabs, you fill in some little questionnaire and you can knock them out for �15 a tab, so the mark-up is good."

Many people want to buy the pills, he said, from men in their early 20s to old age pensioners.

Some think they have a sexual problem, but others just take Viagra for recreational purposes.

Viagra manufacturer Pfizer warns that Viagra should not be taken unless it is recommended by a doctor.

In combination with nitrates used to treat heart conditions, it can prove fatal.

Fraught with difficulty

Miranda Cavanagh, head of public affairs for Pfizer in the UK, said: "This is an area that is fraught with difficulty because most of the sites from which you can order prescription drugs are beyond the jurisdiction of UK law, so it is an area that is very difficult to regulate.

"We only want the medicines we produce to be used responsibly, and under the guidance of a doctor."

Dr George Rae, chairman of the British Medical Association's GP prescribing sub-committee, said: "We have got to be looking at the principle of patients being able to get access to medication over the internet without the safety of a professional being involved.

"If we start moving away from that safe environment to being able to access medication over the internet and from far afield, I think that one is going to find that there are going to be one or two catastrophes as a result thereof."

Health minister Lord Hunt said he was worried by the potential risk to people who were taking Viagra without medical supervision.

He said the law relating to selling medicines on the internet was unclear.

"We have officers who are fully trained in internet search techniques, and they are investigating a number of incidences of advertising on the internet."

Lord Hunt said officials were also investigating whether it was feasible to safely sell prescription medicines on the internet.