The government has awarded contracts to double supplies of antiviral drugs to treat pandemic flu.
The Department of Health said that by April, when the extra drugs were delivered, there would be enough to treat everyone who is predicted to fall ill in a pandemic. The "worst-case scenario" is that an outbreak could affect half the population of the UK.
The contracts have been awarded to Roche and GlaxoSmithKline. They will result in an additional 7.6 million treatment courses of Tamiflu and 10.6 million treatment courses of Relenza.
Public health minister Dawn Primarolo said: "Antivirals are an important part of our robust countermeasure strategy and will ensure we respond effectively in the event of a flu pandemic."
World Health Organisation assistant director-general for health security and environment David Heymann said the UK is among the best prepared countries in the world.
"Purchasing enough antivirals to treat the worst case attack rate scenario, and two different antivirals in case resistance develops, is a first class example of the UK's advanced planning," he said.