Shares in the company sank 25 percent at opening in London.

Indivior has been engaged in a long-running fight against the launch of cheaper equivalents of its opioid addiction treatment by competitors including India's Dr.Reddy's Laboratories and Teva Pharmaceuticals.

The company said it would file an emergency motion to prevent a mandate under the decision by the U.S. Court of Appeals for the Federal Circuit taking effect on Feb. 11.

Failing that, it said it expected both Dr. Reddy's and another competitor, Alvogen Pine Brook LLC, to relaunch their treatments, resulting in potentially huge losses in market share for Subloxone.

"We acknowledge that the Company faces major disruption in the immediate future from a potential material and rapid loss of market share," Chief Executive Officer Shaun Thaxter said.

He said the company had implemented the initial stages of a contingency plan outlined in December, leaving it with $920 million in cash and equivalents at the end of the 2018 financial year.

(Reporting by Pushkala Aripaka in Bengaluru)