The UK's biggest drug maker, GlaxoSmithKline (GSK), is set for a showdown with the US Food and Drug Administration (FDA) over its bestselling diabetes pill Avandia.

A 'make-or-break' scientific panel convened by the FDA will meet in Washington, marking the climax of a heated three-year debate on whether the daily-dose drug increases patients' risk of having a heart attack or stroke.

GSK has also been accused of trying to intimidate scientists researching the risks of the drug.

The drug-maker fiercely defends what was once its second-biggest selling product. It has produced studies suggesting Avandia is safe, but the pill's sales have halved from £1.4bn in 2006 to £771m last year.

FDA scientists are sceptical of the quality of the GSK study, which showed Avandia users were no more likely to have heart problems than other diabetes patients.

A ruling against GSK could result in the product being pulled from the US market and implicate the company in lawsuits worth billions of dollars.

The European Medicines Agency is also reported to have launched a review into Avandia's risks and benefits.