More than one anti-viral drug may be needed in a pandemic
A second anti-viral drug is being stockpiled to treat people in the event of a flu pandemic, ministers say.
The government has bought 10.6 million doses of Relenza in addition to another 7.6 million doses of Tamiflu - the drug that has been stockpiled since 2005.
It comes after a UK study last year raised concerns about resistance to Tamiflu in the H5N1 strain of bird flu which has spread in south east Asia.
The UK will now have enough anti-viral drugs to treat half the population.
To date there have been 403 human cases of the H5N1 bird flu virus and 254 deaths, mostly in south east Asia.
However the virus cannot easily pass from human to human at present.
So far, many of those who have been infected have been poultry workers who have come into intimate contact with infected birds.
Experts warn that if the virus acquires the ability to pass from human to human, then it will pose a potential threat to millions across the globe.
Without antiviral treatment, estimates suggest that up to 750,000 people could die in the UK during a pandemic.
Tamiflu and another drug, Relenza, work by inhibiting a key part of flu called neuraminidase (N1) which is responsible for the release of the virus from infected human cells and allows the disease to spread.
The UK had previously stockpiled 14.6 million doses of Tamiflu - enough for a quarter of the population.
But in 2006, leading scientists warned that more than one anti-viral drug should be stockpiled because of potential resistance.
Research published in the journal Nature last year also concluded that no single drug would be enough to treat all the victims of a global flu pandemic.
The Medical Research Council study characterised a mutation in the structure of N1 that has been observed in human cases of H5N1.
They found then when this mutation occurred the virus became resistant to Tamiflu, while still remaining susceptible to Relenza.
The same mutation was also found to produce resistance in seasonal influenza.
Once the new arrangements are in place - which will take a few months - there will be 33.5 million doses of anti-virals available, the Department of Health said.
Public health minister, Dawn Primarolo, said: "The UK is already widely recognised as one of the best prepared countries in the world.
"The increased flu-drug stockpile means that we should be able to treat everyone who falls ill in a pandemic."
Professor John Oxford, an expert in virology at Queen Mary, University of London said it was sensible to have a safety net.
"It's very reassuring to have Tamiflu as we know you do a lot better with Tamiflu even if it's given very late.
"There's not much drug resistance to H5N1 at the moment but if it becomes a problem then we need an alternative and that's Relenza."
Dr Steve Gamblin who led the research on Tamiflu resistance said he was pleased that Relenza was being stockpiled.
"Our science suggests that it would be a wise step to have stockpiles of both available antivirals to combat an H5N1 pandemic."