(Makes clear Greece not selling entire OPAP stake in last paragraph)
BRUSSELS, Jan 24 (Reuters) - Europe's highest court said on Thursday that OPAP's current gambling monopoly in the Greek market was illegal, unless the authorities in Greece set strict conditions to protect consumers.
The European Court of Justice ruled against the monopoly in a challenge by rivals William Hill, SportingBet and Stanleybet.
EU law did not allow for national legislation granting the exclusive gambling rights to a single company, unless the authorities were genuinely reducing access to gambling and controlling expansion of the sector to combat criminality, the court said in a statement.
Judges at the Luxembourg-based court had in previous cases found problems with gambling monopolies in Italy, Germany and Austria.
The judges gave Greece two options.
They said that if market liberalisation would not allow for the level of consumer protection it wanted, the Greek government could undertake "reforms of the monopoly and make it subject to effective and strict controls by the public authorities".
But if Greece chose to liberalise the gambling market - which is not compulsory under EU law - it would have to be transparent and treat local and foreign operators equally.
Athens is selling 33 percent of OPAP, almost its entire stake, as part of a privatisation drive agreed with its international lenders. (Reporting by Foo Yun Chee; Editing by Rex Merrifield)