The decision is the fifth to go in favour of Lloyd's in the past 18 months. Lloyd's says it will leave the provisions it has made for the claims in place until the appeals process has been exhausted, but the total saving could be several hundred million dollars.
The claim, by the aerospace contractor Martin Marietta, had been spread right across the Lloyd's market. In 1986 it began litigation to claim the dollars 180m from insurers who up to 1972 had written policies covering it.
A Los Angeles jury in California's Superior Court found that the firm had concealed material information about pollution it had caused - particularly a toxic chemicals spill near Denver in 1957 and test missile firing in Colorado - so the policies were void.
Other insurers had settled out of court with Martin Marietta before the trial. Lloyd's has more than 150 similar US pollution cases in the pipeline.