Roche, makers of Tamiflu, said up to 50,000 courses were used by the NHS in an average flu season and that most of those had already gone, even though there was little flu this year.

"People have been panic buying from doctors and pharmacies because of the fear about avian flu. We do have stock in place but because there has not been much flu about this year we have not pushed it out," a spokeswoman said.

Bupa, the private health organisation, said it had been asked to order stocks of Tamiflu for some of its corporate clients.

"We will be allocated the drugs by the manufacturer once they have fulfilled all their essential orders. All essential orders and allocation are managed by the manufacturer," Bupa said in a statement yesterday.

Tamiflu was developed as a treatment for seasonal winter flu and can shorten the illness and reduce its severity if taken within 48 hours of the onset of symptoms. It also works against avian flu, reducing the risk of complications such as pneumonia, but evidence of its effectiveness against a pandemic is limited.

As Tamiflu is a prescription-only drug, companies cannot hand out packs to their employees for protection in a pandemic. The drug must be stored in licensed premises and dispensed with a prescription - in effect limiting it to companies with a doctor or occupational health service.

Roche said it was planning for a more severe flu season this winter because the last two had been relatively mild, with low rates of flu.

The Government has ordered 14.6 million courses of the drug to prepare for a pandemic, at a cost of £200m, which is being delivered at the rate of 800,000 courses a month. Front-line NHS workers would be given priority for the drug, along with other essential workers including the police, fire service and armed forces.

So far, 2.5 million courses have been delivered but the full order, which will cover a quarter of the population, will not be complete until December next year.

The Roche spokeswoman said: "Our priority is to fill the Government's order, which should cover the general public as well. We are in discussion with a number of corporations about supplying them direct but they have requested that their details are kept confidential."

The private medical organisation Casualty Plus, which runs nine walk-in medical centres across London, said it had seen a 12-fold increase in demand for seasonal flu vaccinations and had prescribed 120 packs of Tamiflu to private patients over the weekend.

Many companies had staff who travelled overseas and needed the drug for trips to the Far East. "We are writing private prescriptions for Tamiflu as the clinical need dictates," a Casualty Plus spokesman said. "We have been approached by companies who have asked us to hold stocks of Tamiflu for them. We are looking into the ethical and practical implications of this."

A World Health Organisation spokesman said: "We do not advise individuals to stockpile this drug for any purpose. There is no indication for anyone at this point to be taking the drug other than the specific high-risk groups."