Academic group has warned of 'substantial problems' with trials of the drug stockpiled by governments around the world in 2009

A senior cross-party group of MPs is calling for a parliamentary investigation to establish whether the NHS spent £500m on the Tamiflu drug without a proper assessment of its effectiveness.

The MPs, led by the former shadow home secretary David Davis and the Tory GP Sarah Wollaston, have asked the chair of the powerful commons public accounts committee, Margaret Hodge, to establish whether evidence about the drug was concealed.

Hodge, who recently shamed Starbucks into paying more tax, was asked to intervene after concerns were raised about the effectiveness of the anti-flu drug manufactured by the Swiss pharmaceutical group Roche. The Cochrane Collaboration, an academic group, has warned of "substantial problems" with trials of the drug which was stockpiled by governments around the world at a cost of billions of pounds during the 2009 flu pandemic.

In the letter to Hodge, written by Wollaston and signed by the Labour MP Valerie Vaz, the cross-party group wrote: "We are writing to you in your capacity as chair of the public accounts committee to ask if your committee would consider conducting an inquiry into the waste of NHS resources caused by the concealment of trial data for drugs, medical appliances and implants."

The MPs pointed out that the last government spent £500m on the drug – 0.5% of the entire budget of the NHS – which may have been "no better than a placebo". They added that Roche has conducted more than 123 clinical trials, of which at least 60% remain unpublished.

The letter to Hodge adds: "In the event that your committee finds Roche has deliberately concealed evidence which shows Tamiflu to be simply a placebo with side effects, you might wish to consider a recommendation that the government seeks repayment of the £500m cost."

A spokeswoman for Roche insisted that many trials had shown that Tamiflu had a "clinically meaningful benefit for patients". But the company announced it would establish a multi-party advisory board to review the "totality of Tamiflu data".

The spokeswoman said: "We stand behind the robustness and integrity of our data supporting the efficacy and safety of Tamiflu. Numerous clinical trials and real-life medical experiences have clearly demonstrated the clinically meaningful benefit for patients one year of age and older, for both the treatment and prevention of influenza.

"Roche has made full clinical study data available to health authorities around the world required for their review as part of the licensing process. It is the role of global health authorities to review detailed information on medicines when assessing benefit and risk. Almost 80% of Roche's Tamiflu clinical data has been published as primary publications or made available to the scientific community on the web as complete core reports. Of the remaining data, most come from studies that have recently been completed. Roche is working to make them publicly available.

"Tamiflu is effective in reducing the severity and duration of symptoms in those infected with the flu and decreasing the risk of getting the flu. Analyses by global health authorities and independent groups of researchers, such as the Harvard School of Public Health, have supported these findings. After the 2009 H1N1 'swine flu' pandemic a wealth of data has provided further assurance that Tamiflu is effective and well-tolerated and reduces the risk of complications."

The Cochrane Collaboration highlighted its concerns about the drug in October. It wrote: "The authors have been unable to obtain the full set of clinical study reports or obtain verification of data from the manufacturer of [Tamiflu] oseltamivir (Roche) despite five requests between June 2010 and February 2011. No substantial comments were made by Roche on the protocol of our Cochrane Review which has been publicly available since December 2010. Based on our assessments of the documents we could obtain, we came to the conclusion that there were substantial problems with the design, conduct and availability of information from many of the trials."