* Commission seeks to enforce single energy market
* Polish infringement case dates back to June 2009
* Polish economy ministry says ready to defend its stance (Updates with Polish reaction)
BRUSSELS, June 20 (Reuters) - The European Commission has referred Poland to the Court of Justice of the European Union, saying the country's regulated gas prices for business consumers are in breach of EU rules.
The Commission, the EU executive, has stepped up action to enforce its single energy market, which aims to increase consumer choice, improve security of supply by increasing cross-border energy flows and make energy prices more competitive.
"Competitive energy markets will offer citizens and businesses secure and sustainable energy supplies at lowest possible cost," EU Energy Commissioner Guenther Oettinger said in a statement on Thursday.
In a statement, the Polish economy ministry said it would "defend (its) stance in the Court of Justice if the Commission does not withdraw its decision".
It said it supported a liberalised single market, but the Commission needed to take account of the situation in particular member states".
The Commission says regulated prices can limit competition and distort markets because they do not reflect demand.
The case dates back to June 2009, when the Commission opened an infringement procedure against Poland, which still is not complying despite numerous exchanges, it said.
Poland could face daily fines, depending on the European Union court's decision.
Poland controls prices that corporations and private consumers pay for gas from state monopoly PGNiG.
Its energy market regulator URE said it would in July introduce further measures following on from liberalisation steps taken on wholesale and trade in liquefied natural gas.
Ireneusz Lazor, chief executive of POLPX, the Polish power exchange, said the Commission reaction was "an alarm calling" to speed up liberalisation.
The exchange launched gas trading at the end of 2012. (Reporting by Barbara Lewis in Brussels and Agnieszka Barteczko in Warsaw; editing by Jeff Coelho)