GlaxoSmithKline pledged to fight a tax claim that could cost the pharmaceuticals company as much as £2.9bn.
The group, which was formed from the merger of Glaxo Wellcome and Smith-Kline Beecham in 2000, has received a bill for £1.5bn from the US Inland Revenue Service in relation to taxes on profits at Glaxo Wellcome between 1989 and 1996.
The interest bill on a successful claim by the IRS would be in the region of £1.4bn, the company said.
The disputed bill , which would come on top of taxes of $1.3bn already paid during the seven year period, is likely to be heard in the US Tax Court next year or in 2006.
The company said: "GSK considers that the additional tax claim by the IRS on Glaxo heritage products is inconsistent with the treatment of other pharmaceutical companies, including GSK legacy company SmithKline Beecham."
Glaxo said it tried to resolve the dispute by referring the matter to negotiations between the US and UK tax authorities.
It said: "The company believes these discussions collapsed when the UK supported the GSK position that no additional taxes were due to the IRS. The company continues to believe, based on external professional advice, that it has made adequate provision for tax liabilities."
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