GLAXOSMITHKLINE (GSK) has vowed to fight a US court ruling requiring it to hand documents to investigators looking into alleged collusion between the world’s big pharmaceutical groups.

The ruling, given last Friday, compels the UK drugmaker to turn over to Mike Hatch, Minnesota’s attorney-general, papers relating to its policy of limiting drug sales to Canada.

Mr Hatch is investigating whether GSK conspired with pharmaceutical rivals, including Pfizer, AstraZeneca, Eli Lilly and Wyeth Laboratories, when it moved to restrict sales to Canada in an attempt to prevent its medicines being re-imported and resold in the US.

Parallel trade of drugs between Canada and the US is a big problem for drugmakers and costs the industry billions of dollars a year. The practice refers to traders who buy branded drugs from Canada, where they cost between 30 and 80 per cent less because of government-imposed price controls and import them into the US. Drug companies argue that they cannot lower prices in the US because revenues from sales there fund critical research programmes.

The ruling, by the Minnesota district court, requires GSK to hand over documents held in Britain and Canada. The drugmaker has already passed Mr Hatch documents held in the US.

Mr Hatch said he hoped to find proof of collusion among drug groups after rival companies announced similar efforts to block importation into the US of drugs from Canada soon after GSK’s action. “We think that’s too much of a coincidence,” he said.

GSK yesterday said that its action in restricting drug sales to Canada was “perfectly legal”. The company said: “The attorney-general is acting to encourage (US) sales of prescription drugs from Canada, which is illegal.”

GSK said that US officials had “significant evidence” that illegal trade put patients at risk because the medicines could be contaminated, counterfeit, outdated or wrongly labelled.