LONDON, Aug 19 (Reuters) - AstraZeneca said on Tuesday the U.S. government was closing a probe into a major clinical trial used to win marketing approval for its new heart drug Brilinta and no further action was planned.

The decision last October by the U.S. Department of Justice to investigate the study cast doubts over the future of the medicine, which AstraZeneca argues is a potential $3.5 billion-a-year seller but which has so far registered only modest sales.

AstraZeneca flagged up the promise of Brilinta in its defence against an abortive $118 billion takeover bid by Pfizer earlier this year.

(Reporting by Ben Hirschler; Editing by David Holmes)