Production of the drug identified as the prime weapon against bird flu is being stepped up, its makers have said.
Enough Tamiflu has been ordered for 25% of the UK population
The drug company Roche plans to double Tamiflu production by the end of this year, and again by mid-2006.
The plans follow World Health Organization advice to governments to stockpile the drug.
Indian drugmaker Cipla has also claimed it is to start manufacturing a generic version of Tamiflu for use in developing countries.
Cipla Chairman, Y.K. Hamied, told Reuters: "We have finalised a process a few weeks ago, and now we are starting manufacture."
However Roche has said the 10-step year-long production method of Tamiflu was too complex to be outsourced to other companies to boost supplies, and it would take other companies up to three years to be ready to produce the drug.
It has so issued no direct comment on Cipla's announcement.
But a Roche spokesman said: "We fully intend to remain the sole manufacturer of Tamiflu, together with our partners.
The deadly H5N1 strain of avian flu was discovered this week in birds in Turkey, and birds in Romania have also been found to have some form of the virus, although it is not yet confirmed to be the same strain.
H5N1 has killed millions of birds across Asia and infected 116 people, killing more than 60 of them.
Scientists fear that the virus, known to pass to humans from birds, could mutate and start to spread easily from person to person.
Countries, including the UK, are stockpiling doses Tamiflu - which is an anti-viral drug, not a vaccine - to use if avian flu mutates and spreads between people.
Roche is under pressure to increase production of Tamiflu amid fears there would be a shortage in the event of a bird flu pandemic.
Last week, UN Secretary-General Kofi Annan said the usual patent rules may have to be suspended if there is an outbreak of the disease so that other companies could be allowed to make the medicine as well.
Roche said it did use other firms to help with certain stages of the drug's production.
A spokesman said: "We are already collaborating with several specialist companies on the production process for Tamiflu.
"This has nothing to do with the patent."
And Alexander Klauser, from the company, told the Associated Press: "We asked governments several years ago to make Tamiflu orders for pandemic purposes well in advance.
"We explained the procedure to them, how it works and that we had to start production well in advance or we wouldn't be able to produce Tamiflu in the required amounts on time."
Industry analysts expect the sudden demand for Tamiflu to generate windfall profits for Roche, although the company is also donating three million packs of Tamiflu to the WHO for use anywhere in the world.
David Salisbury, head of immunisation for the Department of Health in England, said: "We are getting 800,000 more doses of Tamiflu every month, so we will have the level we believe appropriate if we have a pandemic in this country.
"The US will have something like one twenty-fifth of what we're building up in our stockpile."