DRUGS firm GlaxoSmith- Kline (GSK) is to fight a judge's ruling that could dent sales of its leading product.

Last year, the anti-depressant treatment Paxil recorded sales of £2.055bn worldwide, but its reign could be endangered after the US District Court for the Northern District of Illinois ruled that a patent on the drug had not been infringed by a rival pharmaceutical firm. That decision potentially exposes GSK's market dominance in the US.

The pharmaceutical industry is dogged by litigation, with a constant stream of challenges by generic producers against patents obtained by large corporations carrying out the research and development of leading drugs.

On this occasion, TorPharm Pharmaceuticals, a subsidiary of Apotex, tackled GSK, saying its anti-depressant did not breach one patented type of Paxil because it was produced using significantly different means.

The US court decided that while Apotex had not broken the patent rules and would be allowed to bring the drug to market, GSK's patent on that type of Paxil still stood.

GSK, which has a plant in Barnard Castle, County Durham, has vowed to appeal against the decision.

It is not clear when Apotex will be able to launch its generic version of Paxil.