ATHENS (Reuters) - British sports betting operator Stanleybet has threatened to sue Greece for lost revenue in a long-running legal battle over the monopoly of gaming firm OPAP (>> Greek Org of Football Prognostics S.A.).

Stanleybet accuses Greece of tacitly rejecting its 2004 request to offer sports betting in the country and said on Wednesday it had told the government and OPAP it will seek damages of about 240 million euros.

OPAP, one of Europe's biggest gaming companies, has a monopoly in offline sports betting and lotteries in Greece until 2030.

Athens agreed earlier this month to sell a controlling stake and management rights in OPAP to Greek-Czech investment fund Emma Delta.

"The Greek authorities seem resolved to maintain an illegal framework benefiting a company that is not 99 percent private," Stanleybet's Chief Operating Officer John Whittaker said in a statement.

"With this action (Stanleybet) is legitimately stating its intention to claim its legal rights to be compensated."

The finance ministry was not immediately available for comment.

Stanleybet and its UK rival William Hill (>> William Hill plc) have already brought the case to a Greek court and the trial is expected to start in October.

The European Court of Justice ruled in January that OPAP's monopoly is illegal, urging Greece to push ahead with reforms to protect consumers or open up the sector.

Greeks are among Europe's most ardent gamblers. In the face of a protracted recession in the debt-laden country, OPAP's revenues were about 4 billion euros last year.

The Remote Gambling Association and the European Gaming and Betting Association have filed a complaint to the EU Commission saying Greece is breaching EU laws by failing to authorise online betting.

(Reporting by Angeliki Koutantou; Editing by David Cowell)