In the letter, EU industry Commissioner Stephane Sejourne said Brussels was gathering information to assess if KHNP received "foreign financial contributions" that tilted the playing field in its favour.

This was a preliminary move that could lead to the opening of an official investigation.

"A contract creates final facts which would jeopardise the Commission's right to carry out its investigation effectively and its ability to ensure that entities take effective remedial action," Sejourne wrote, asking Prague to postpone the signing.

Sejourne's letter was sent on May 2, ahead of a signing ceremony originally planned five days later with KHNP, which won the contract last July after beating France's EDF in the tender.

In the interim, a Czech court last week blocked the contract until a verdict is reached in a lawsuit filed by the French giant.

KHNP is due to build the two units at the southern Czech nuclear plant of Dukovany, run by the state-run CEZ group.

The Czech Republic, an EU member of 10.9 million people, relies on nuclear power for 40 percent of its electricity.

With the two new units and small modular reactors due to be built by 2050, the share of nuclear energy is expected to rise to 50 percent as the country shifts from burning fossil fuels.

CEZ CEO Daniel Benes on Monday said Prague should "reject" the commission letter, telling national news agency CTK: "The French will do everything to prevent such a power plant from being built".

The commission rejected accusations that nationality -- Sejourne is French -- had anything to do with it.

"It's not a French commissioner defending French interests. To the opposite, it's a college member enforcing the legislation and engaging with the Czech authorities to protect our single market," EU spokesman Thomas Regnier told a press conference.

KHNP has offered to build the two new units for around 200 billion Czech koruna ($9 billion) each.

CEZ expects construction to begin in 2029, with the first new reactor launched in trial operation in 2036.

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CEZ