The company has not yet decided whether this would involve a fresh legal action or taking part in the regular reviews Mr Dobson promised when he placed the drug on a restricted list.

Success in any challenge could lead to further embarrassment for the government.

On Wednesday the company won a High Court action declaring Health Secretary Frank Dobson's interim guidance on the drug unlawful.

A senior Pfizer figure confirmed on Thursday that the company would seek to have the permanent restrictions on NHS use of the drug relaxed.

The High Court ruling also meant that Pfizer could now sue the government for millions of pounds worth of damages in lost profits.

Guidance restricts drug

Mr Dobson first restricted the drug when it was licensed for use in September 1998. He wrote to doctors asking them only to prescribe the drug on the NHS in "exceptional circumstances".

Frank Dobson will take the new guidelines before Parliament

Mr Justice Collins ruled on Wednesday that the letter interfered with doctors' legal duty to prescribe what they saw as the best treatment for a patient's condition, and as such broke both UK and European law.

However, before the case came to court, the Department of Health introduced specific guidelines for NHS prescribing of the drug. Subject to Parliament's approval, these will become law in July.

The full guidance placed Viagra on the so-called Schedule 11 - a list of drugs restricted on the NHS.

Although this increased the number of patients eligible for Viagra from 15% to 17% of the impotence sufferers in the UK, Pfizer argues this is still too restrictive.

'Discriminatory and unfair'

Andy Burrowes, marketing manager for Pfizer, said: " As far as we are concerned on the Schedule 11 issue, we do believe the proposals are discriminatory and unfair.

"We will seek to have the situation changed so every man with erectile dysfunction can get Viagra."

The company would make a "relatively quick" decision on whether or not to sue for damages, he said.

"At the moment we are still considering the full judgement line by line. We're not ruling anything out."

'The situation has changed'

Following the ruling, Mr Dobson said: "This judgement was about the interim guidance to GPs.

"The judgement has made it clear that life has moved on and doesn't relate to what we have been doing since then.

"The procedure we have followed in developing the new criteria for Viagra on the NHS announced on 7 May is in line with the judgement."

But a spokeswoman for the Impotence Association said: "The government's proposals still remain discriminatory and restrictive with only 17% of sufferers able to get treatment on the NHS."

First impotence treatment restricted

Until Viagra was licensed, the NHS funded all impotence treatments where clinically necessary.

However, restrictions on other anti-impotence treatments will come into force on 1 July if Parliament approves the new guidelines.

The following men will qualify for anti-impotence treatments on the NHS:

• Men with diabetes
• Men treated for prostate cancer or who have had their prostate gland removed
• Men treated for kidney failure, by transplantation and dialysis
• Men with multiple sclerosis or other single gene neurological diseases that cause impotence
• Men who have had polio
• Men who have had polio
• Men with spina bifida
• Men with spinal cord injuries
• Men with Parkinson's disease
• Men with severe pelvic injury or who have undergone radical pelvic surgery