The world's largest online gambling operators asked the European Union on Tuesday to investigate a new Greek gaming law, saying it violates the bloc's free market rules to the benefit of the country's sports betting monopoly OPAP.

Cash-strapped Greece last month passed a bill setting rules for online betting operators in a bid to capture a bigger share of the country's 19 billion euro gambling market, much of which currently operates outside the law.

But the legislation also awards OPAP an exclusive license to operate a new breed of gaming machines, called video-lottery terminals (VLT), as part of the debt-laden country's plan to raise 4 billion euros from asset sales this year.

RGA, an association of online gambling firms, complained to the European Commission that the law's tax provisions favored OPAP, making it harder for online operators to obtain licenses in Greece.

Under the new law, OPAP's off-line gambling operations are exempted from a 30 percent gross profit tax imposed on online operators, according to the RGA whose members include the UK's Ladbrokes and William Hill.

We are fully aware of the fiscal pressures on the Greek authorities at present, but they do not justify the imposition of anti-competitive tax provision which benefit the existing monopoly gambling provider, RGA Chief Executive Clive Hawkswood said in a statement.

Greece's stated purpose with the gaming law was also to boost OPAP's market value, currently at about 2.3 billion euros, to maximize the proceeds from the planned sale of a stake of up to 34 percent stake to private investors later this year.

We urge the European Commission to investigate the new legislation for being in breach of state aid rules, Hawkswood said.

Betfair had also filed a complaint to the EU in June, saying that a blanket ban the country imposed on betting exchanges violated competition rules.

(Reporting by Angeliki Koutantou; Editing by David Cowell)