The Department of Health put a block on NHS prescription of the drug in September amid fears that excessive demand could cost the health service more than £1bn a year.

GPs were told they could only prescribe the drug in "exceptional circumstances" until ministers issued further guidance.

However, the British Medical Association has warned that some GPs are already routinely issuing prescriptions for the drug.

They have warned that, if the Department of Health does not issue definitive guidelines within the next month, they will tell all GPs that they should go ahead and routinely prescribe the drug where necessary.

Dr Ian Bogle: GPs have no choice

BMA chairman Dr Ian Bogle has written to Health Secretary Frank Dobson warning that GPs have no alternative but to prescribe Viagra because they were bound by their terms of service to do the best for their patients.

Dr Bogle writes: 'Doctors have been placed in an impossible position: on the one hand they are mindful of the Department's request not to prescribe Viagra but on the other have to respond to the needs of their patients.

"Indeed, to refuse to prescribe this licensed medicine on a prescription form when it is clinically necessary would leave GPs open to accusations of breaching their terms of service."

Dr Bogle said the Department of Health must address the general issue of how expensive new medicines should be prescribed by the NHS.

He said it was inevitable that the NHS would be faced with more and more tough decisions as medical technology allowed for the development of ever more sophisticated drugs.

Tough decisions

Dr Howard Stoate: we cannot have a free-for-all

Labour MP Dr Howard Stoate, a former GP and member of the Health Select Committee, said the NHS could not afford to fund all expensive treatments, and some tough decisions about priorities had to be made.

He said: "All NHS services must set priorities. We cannot simply have a free-for-all because that would unbalance the drug budgets signficantly.

"If Viagra were prescribed freely without hindrance there would be some very, very unfortunate side effects, in that some other drugs might not be so available, other treatments might not be so available.

"It is a matter of fitting this into the overall priorities of the NHS.

"The government is going to have to make some hard decisions. It is going to have to decide which patients and which conditions can have it and which can't."