GlaxoSmithKline is to fight a US court ruling which invalidates patents for its Augmentin antibiotic drug.
Teva Pharmaceuticals Industries, Geneva Pharmaceuticals and Ranbaxy Laboratories sued to invalidate three patents that protected Augmentin from generic competition until the end of this year.
Augmentin currently generates sales of £1.376bn (€2.160bn) a year.
GSK adds that a lifting of its exclusive rights would force it to reduce earnings estimates for this year and next.
It wants to retain exclusive rights to Augmentin until the end of the year, buying time to get new products ready for introduction in 2003.