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UPDATE 1-London-listed drugs firm Indivior suffers setback in U.S. patents ruling

ruling@ (Adds share price fall, background about Indivior)

Sept 1 (Reuters) - Indivior said on Friday it would appeal against a U.S. court ruling that generic drug maker Dr Reddy's had not infringed its patents, potentially opening the way to a rival to the firm's Suboxone Film opiod addiction treatment.

London-listed Indivior has said it would not be able to rely on patents to prevent Dr. Reddy's from making and marketing a generic alternative to Suboxone Film in the United States, unless the court's ruling was reversed on appeal.

The firm said the average market share of Suboxone Film in the United States was 61 percent in 2016, and accounted for 80 percent of Indivior's total revenues last year.

Shares in Indivior, which was spun off from Reckitt Benckiser in 2014, were down 38 percent at 259.5 pence($3.35) after the open, after the ruling by the U.S. Delaware District Court.

Indivior said in a statement it could not quantify the precise financial impact of generic alternatives to Suboxone Film on revenues but said it "could potentially result in a rapid and material loss of market share for Suboxone Film in the U.S."

Indivior said if pharmacies could substitute Suboxone Film with a generic rival without direct consultation with the patient it could lead to the British firm's treatment losing up to 80 percent of its market share "within a matter of months".

Indivior said that as of Aug. 29 the U.S. Food and Drug Administration had not announced it had granted tentative or final marketing authorisation to any generic Suboxone Film alternative.

If the FDA grants approval to Dr. Reddy's, the British firm said its rival would be able to market a generic alternative to Suboxone Film in the United States.

Indivior said in June that it had won a U.S. patent battle against Actavis and Par Pharmaceutical over Suboxone Film.

($1 = 0.7746 pounds) (Reporting by Justin George Varghese in Bengaluru; Editing by Edmund Blair)