In October, Prague submitted a support scheme for the project to build two new nuclear reactors at the country's southern Dukovany plant.

The plan would see the government grant a low-interest loan of between 23 billion and 30 billion euros ($27-35 bn) to cover construction costs.

The government has also proposed setting up a two-way contract for difference (CfD) that would guarantee a minimum price for the electricity produced by the plant for 40 years.

This would ensure stable revenues for the plant's operator, the EDU II consortium in which the government has an 80-percent stake.

While the aid "facilitates the development of an economic activity", the commission is concerned the guaranteed price mechanism is not "fully in line" with the EU state aid rules, according to a Monday statement.

The Czech Republic signed a 16-billion-euro contract in June with South Korea's KHNP to build the two reactors.

Construction is slated to begin in 2029, with the first unit scheduled to start operating in 2036.

It is a historic investment for the central European country of 10.9 million inhabitants, which is turning away from coal and has become fully independent of Russian oil and gas.