Demand for sex drugs and heart pills is fuelling an explosion in illegal internet pharmacies, a report reveals.

Tens of thousands of people are risking their health and money by buying Viagra and cholesterol-lowering pills online.

It raises fresh fears that potentially dangerous medicines are being used as 'lifestyle' enhancers by people who never seek advice from a doctor.

Britain is now the second-largest host of unlicensed internet chemists - with 18 per cent of the market - after America, according to researcher MarkMonitor.

Most are unregistered and deal illegally in prescription drugs such as pills for erectile dysfunction, heart disease and stress, it claims.

But the majority of the drugs are thought to be fake or out of date.

The report also said more than half of the sites do not offer secure financial transactions, putting thousands at risk of identity fraud.

MarkMonitor, which investigates threats to global brands, said the web boom was worth at least £2 billion annually for six drugs alone.

They tracked the sales of the most popular drugs, believed to include Lipitor for lowering cholesterol and Viagra for erectile dysfunction.

Each of the sites had as many as 32,000 visitors every day, buying prescription drugs at a fifth of the high street price.

Charlie Abrahams, chief executive in Europe, said: "These illegal pharmacies are probably expanding at a rate of 40 per cent to 50 per cent a year. The worry is that most of the drugs appear too cheap to be anything but fakes."

The Royal Pharmaceutical Society is piloting a logo scheme to identify all bona fide internet chemists.