After a lengthy fight, Swiss drugmaker Roche said it had agreed to hand over data from all clinical trials of its best-selling flu drug Tamiflu to a group of outside researchers.

Tamiflu has been approved by regulators worldwide and stockpiled by many governments in case of a pandemic, but some scientists claim there is little evidence it works and have lobbied since 2009 for Roche to release all its trial data.

Sales of the drug hit close to $3 billion in 2009 due to the H1N1 swine flu pandemic, although they have since declined.

In an email to the Cochrane Collaboration, a non-profit group that reviews trial data to assess the value of drugs, Roche said it would provide clinical study reports on all the 74 studies into its medicine, over the next few months.

A copy of the April 2 email was supplied to Reuters on Thursday.

Roche said it would edit the study reports to ensure patient confidentiality and protect legitimate commercial interests, adding that handing over the information would take time since some of the reports ran to thousands of pages.

Campaigners who have been pushing pharmaceutical companies to be more open said they were pleased Roche had finally done the right thing, although they argued it should not have delayed access to the data in the first place.

"It shouldn't have taken the researchers years of persistence and publicity to get these Tamiflu results," said Sile Lane, director of campaigns at Sense about Science.

The Cochrane group gave Roche's move a cautious welcome but its researchers said they were still concerned that editing of the data and other problems might make analysis and interpretation difficult.