The treatment, officially still banned from prescription on the NHS, has been linked with 41 cases of adverse reactions between 1 July and 22 December, the Independent reports.

The reactions ranged from skin rashes to major problems such as heart attacks, the newspaper says.

In 11 cases men taking the drug suffered heart disorders, four of whom died. But there is no indication whether they already had heart disease or were taking other drugs for a heart problem. The fifth death was a suicide.

The cases emerged in reports from doctors to the Medicines Control Agency drug-licensing body.

Viagra was licensed in September but ministers advised doctors not to prescribe it on the NHS, other than in "exceptional" circumstances, until further advice was issued.

While that is still awaited, several thousand patients are estimated to have obtained it on private prescriptions.

The British Mecical Association has warned that the government's failure to issue guidelines is endangering patients and it has threatened to defy the ban on NHS prescribing if new guidelines are not issued by Thursday.

BMA chairman George Rae said: "The side-effects highlighted by these figures could be worse, because the non-availability of the drug on the NHS is driving people to get it over the Internet.

"If doctors get the guidelines it will minimise the risk."

A spokesman for Pfizer, the manufacturer, said there was no proof that a number of deaths reported around the world were caused by Viagra.

"The number of deaths isn't surprising if you think that 85% of people with erectile dysfunction are over 45 and six million prescriptions have been written for the drug and 3 million men have taken it."