BRUSSELS (Reuters) - Greek state-controlled betting company OPAP should not have a monopoly because it is using its protected position to expand, in the face of European policies limiting gambling, an adviser to Europe's highest court said on Thursday.

The European Court of Justice was looking into a case brought against Greece by Britain's biggest bookmaker William Hill, online gaming companies SportingBet and Stanleybet after they were denied gambling licences there.

Greek judges considering the case sought advice from the Luxembourg-based court.

"OPAP seems to pursue an expansionist commercial policy ... Those circumstances ... are in my view manifestly inconsistent with the purported objective of reducing the betting and gaming opportunities in Greece," said Advocate General Jan Mazak, in a legal document, laying out his argument against the monopoly.

"In my view ... the activities of OPAP are neither subject to strict control by the public authorities nor effectively limited by the legislative framework applicable to it," he added.

Mazak's opinion is not binding on the judges of the European Court of Justice, but they follow such advice in the majority of cases. They are expected to make a ruling in the coming months.

The European Commission has over the years urged several countries across the 27-country European Union including Greece, Germany, France, Italy and Sweden, to lift gambling restrictions which breach EU rules on free movement of services.

(Reporting by Foo Yun Chee; Editing by Andrew Heavens)