The makers of Viagra have called the government's original decision to restrict NHS use of the drug "legally indefensible".

Pfizer told the High Court on Monday that Health Secretary Frank Dobson was acting outside his powers when he issued guidance last September to encourage doctors not to prescribe the drug.

Viagra was licensed for use in European Union countries in September, but Mr Dobson, fearful of the cost of the drug to the NHS, told doctors they should only prescribe it in exceptional circumstances, pending further notice.

NHS trusts were also told not to support Viagra being provided on the NHS.

Pfizer says this went against doctors' statutory duties to prescribe according to clinical need and may have been in breach of EU law.

It says the drug can only be legally rationed with parliamentary approval, as the government is now seeking to do.

Professional and legal duty

David Pannick, QC, for Pfizer, told the court: "This circular is legally indefensible.

"Its purpose and its effect was to deter GPs from prescribing Viagra.

"We say GPs have a professional and legal duty to exercise their clinical judgment and to prescribe such treatment with such medicine as a patient needs irrespective of the patient's ability to pay for it."

Since the September guidance was issued, the government has published its views on which patients should receive Viagra.

Last week, it increased the number of men who could get the drug.

They include men with diabetes, prostate cancer and Parkinson's Disease.

Impotence experts say the list is still restrictive and discriminatory.

Mr Pannick said the High Court action could be "a precursor" to further legal action over Viagra.

He is asking the court to decide whether the Health Secretary is right to imply that GPs have no statutory right to prescribe according to clinical need.

Clinical need

Pfizer is said to be considering whether to take action against the government over last week's decision to limit NHS availability of Viagra.

The British Medical Association has asked its members to continue prescribing Viagra on the NHS to anyone who has a clinical need.

But it says that, if the new guidance gets parliamentary approval - as Mr Dobson predicts it will do by July - doctors will have to conform.

The Department of Health, which is expected to deny Pfizer's allegations, feared that excessive demand for Viagra could cost the health service more than �1bn a year.

Mr Pannick said GPs often had to deal with patients who might make false claims, and it was for the doctor to use his professional judgment in each case and prescribe what was appropriate.

He added that some of the reasons given on Mr Dobson's behalf for the Viagra restrictions "simply don't withstand serious scrutiny".

"If this was not a serious matter the reasons would be laughable. They are entirely without justification".