Health officials spent £424 million stockpiling a flu drug that has divided experts over how effective it is, a public spending watchdog found.
Around 6.5 million units of the Tamiflu bought by the Department of Health also had to be written off because it could not guarantee the antiviral had been stored correctly, the National Audit Office said.
That mistake cost taxpayers £74 million - a "shocking example of incompetence" according to Public Accounts committee chairwoman Margaret Hodge.
The NAO began investigating after a number of MPs raised concerns about the decision to stockpile Tamiflu.
It said that while there is a general consensus that the drug reduces the duration of influenza and, in certain circumstances, prevents it, there is less agreement about how effective it is in preventing complications like pneumonia, reducing the need for hospital treatment or lowering death rates.
Of the 40 million units of Tamiflu bought, a quarter were written off. Some 6.5 million of those were ditched before reaching the end of their shelf life in 2009-10 because the department was unable to verify that the stock, distributed to the NHS during that year's pandemic, had been stored correctly.
The department began stockpiling Tamiflu in 2006 in response to the increasing concerns about bird flu.
Overall, £560 million was spent on antiviral medicine - £424 million on Tamiflu and £136 million on back-up drug Relenza - between 2006/7 and 2012/13.
Additional stocks of Tamiflu are due to reach their end of shelf life and be replaced during 2013-14 at a cost of £49 million, the NAO said.
Mrs Hodge said: "It is extremely worrying to find that the Government spent £424 million stockpiling 40 million units of Tamiflu between 2006-07 and 2012-13 despite there being question marks over the effectiveness of the drug.
"Although Tamiflu speeds up recovery times, experts do not agree over its ability to reduce complications and hospitalisations.
"In a shocking example of incompetence, £74 million of taxpayers' money was squandered when 6.5 million units of Tamiflu were written off because of poor record keeping by the NHS. There is simply no excuse for this waste.
"It is important that the National Institute for Health and Care Excellence (Nice) and the Medicines and Healthcare products Regulatory Agency (MHRA) assure themselves that they have all clinical trials information. It is essential, not only for the public purse, but for public safety, that the Department of Health carefully examine how it stockpiles medicines in the future."