GlaxoSmithKline, the UK pharmaceuticals group, has vowed to fight a class-action suit brought against it in the US after its share price fell on the back of safety concerns over its blockbuster diabetes drug Avandia.

Kaplan Fox & Kilsheimer, a New York law firm, said it had filed in the US District Court for the Southern District of New York against the company and some executives for allegedly "publicly issuing a series of false and misleading statements regarding Avandia", and urged investors to join the litigation.