* Indivior say can't rely on patents to prevent rival
* Suboxone Film had 61 percent U.S. market share in 2016
* UK-listed firm warns may lose 80 percent of U.S. share (Adds analyst comment, details on rival, updates share moves)
Sept 1 (Reuters) - Shares in Indivior plunged 40 percent on Friday after it said a U.S. court ruling could lead to a generic rival to its treatment for opioid addiction that generates 80 percent of the London-listed drug maker's revenues.
Indivior has said it would not be able to rely on patents to prevent Indian firm Dr.Reddy's from making and marketing a generic alternative to Suboxone Film unless the ruling by the Delaware District Court was reversed on appeal.
Suboxone Film had an average market share in the United States of 61 percent in 2016, the firm said in a statement. The treatment accounted for 80 percent of Indivior's total revenues last year, it added.
Shares in Indivior were down about 40 percent at 252 pence at 0832 GMT, after the ruling on patents for the treatment delivered by dissolvable film placed under the tongue rather than by tablet. The fall wipes out this year's share gains.
Indivior, spun off from Reckitt Benckiser in 2014, said 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 that if pharmacies could substitute Suboxone Film with a generic rival without direct consultation with the patient it could lead to the 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 (FDA) had not announced any marketing authorisation to a generic Suboxone Film alternative.
"Indivior says it's hard to gauge the financial impact but its dominant market position could collapse in a matter of months," said Neil Wilson, senior market analyst at ETX capital.
Sales of Suboxone Film were higher in the first half of 2017 on the same period a year ago, but market share fell slightly to 59 percent from 61 percent. Generic rivals in tablet form are already on the market.
In September, 35 U.S. states and the District of Columbia filed a lawsuit against Indivior alleging that it tried to keep generic versions of a drug off the market.
The lawsuit by the states alleged that Indivior took steps to get patients to switch to the oral strip version of Suboxone.
Indivior said in its statement on Friday it would "continue vigorously defending its intellectual property."
Dr. Reddy's, whose shares were up 9 percent at 0832 GMT, bought the abbreviated new drug application (ANDA) for a generic rival treatment from Teva Pharmaceutical Industries in June 2016 for $70 million.
"Dr Reddy's still needs to obtain FDA approval. So even in the best case, launch by Dr. Reddy's is at least a year away," analysts at Credit Suisse wrote in a note.
Indivior said in June 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 and Keith Weir)