The two companies have appealed a decision by the Czech government to hand the contract to South Korea's KHNP.
The competition office (UOHS) temporarily blocked the Czech Republic from signing the deal on Wednesday, before passing its verdict a day later.
"The proceedings on the proposals by the two companies have largely been halted, while in other parts the proposals have been rejected," the UOHS said in a statement.
It added EDF and Westinghouse could now file an appeal with the UOHS chairman, and that the Czech state-run power group CEZ cannot sign the deal with KHNP until a final decision is made in the case.
KHNP beat EDF to win the tender in July, after Westinghouse's bid was ruled out over flaws in January, with both groups appealing the result the following month.
Westinghouse has accused KHNP of using its technology without authorisation, while EDF said it wanted to make sure the selection process was fair and transparent.
CEZ runs two nuclear plants -- Temelin and Dukovany, both in the south of the country -- which produce around 30 percent of total Czech electricity output.
With the two new units and small modular reactors due to be built by 2050, nuclear energy is expected to account for 50 percent of the Czech energy mix as the EU member of 10.9 million people shifts from burning fossil fuels to greener technologies.
KHNP has offered to build the two new units for around 200 billion Czech koruna ($8.5 billion) each.
Prague expects to finalise the deal with KHNP by March 2025, with construction to begin in 2029 and the first new reactor launched in trial operation in 2036.
frj/mmp/rl
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