GlaxoSmithKline yesterday sought to shore up flagging investor confidence in the drug company by more than doubling its share buyback programme to £12 billion.

The aggressive move helped to lift the pharmaceutical giant’s shares despite a 27 per cent plunge in like-for-like second-quarter sales of Avandia, its controversial diabetes drug, which cut into overall profits and revenues.

J. P. Garnier, the chief executive, said: “We want to send a very clear message to investors that, with or without Avandia, we are very confident about our future.” He added that the increased buyback over two years “will not jeopardise” Glaxo’s flexibility to make acquisitions or fight legal challenges in the future.

Avandia is Glaxo’s second-top-selling medicine and generated sales of £1.4 billion last year. However, prescriptions collapsed in May and June after a leading US cardiologist published a report in The New England Journal of Medicine that claimed that the drug increased the risk of heart attacks by 43 per cent.

Overall, GlaxoSmithKline’s pretax profits were little changed at about £1.9 billion, equivalent to earnings per share of 24p. Sales declined 2 per cent to £5.7 billion as Britain’s biggest drug-maker struggled against a weak dollar.

The company also indicated that it could trim its 2007 earnings guidance of growth of 8 to 10 per cent if a US Food and Drug Administration advisory committee, due to meet on Monday, orders a withdrawal of Avandia from the US market or the introduction of stricter labelling and warnings on use of the product.

GlaxoSmithKline has fiercely defended the drug and questioned the methodology behind the New England Journal report.

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