The European Union has opened a probe to establish whether Hungary's financing of two new nuclear reactors constitutes illegal state aid.

The EU's antitrust watchdog is trying to work out whether a private investor would have financed the building of the reactors in the city of Paks the same way as the government.

EU competition commissioner, Margrethe Vestager, said on Monday that her services have "to carefully assess whether Hungary's investment is indeed on market terms or whether it involves state aid."

She said "complex analysis" would be needed to establish the facts.

The move comes days after the EU Commission opened a legal case against Hungary over questions about contracts for extensions to the Paks II nuclear plant.