Drugs giant AstraZeneca has filed a legal case in the US to protect its best-selling schizophrenia treatment.
Seroquel brings in almost 10% of AstraZeneca's revenues
The Anglo-Swedish company has begun the action against an Israeli firm, Teva Pharmaceuticals, for alleged patent infringement of its Seroquel drug.
In September, Teva applied to the US authorities to make a generic version of Seroquel before the patent protecting it expired in 2011.
Seroquel is one of AZ's top earners, with sales of £1.7bn ($2bn) in 2004.
Teva applied to make the generic version of the drug in late September. Under US rules, AZ has 45 days to file a patent infringement suit to prevent it going ahead.
"AstraZeneca has full confidence in and will continue vigorously to defend and enforce its intellectual property rights protecting Seroquel," the company said in a statement, as it filed the case in the US federal district court of New Jersey.
Teva is one of the world's biggest generic drugmakers.
Generic drug manufacturers produce drugs which are exact replicas of branded drugs whose patents have lapsed, making them cheaper to buy and popular with government healthcare programmes.
Shares in AstraZeneca were 20 pence lower at 2,575p in afternoon trade following the news.