Although President François Hollande has given the go-ahead for the project, his own party seems to have developed doubts, as controversy rages in Britain and in France.
The project is so big that failure could threaten the future of EDF, which is 85 percent state-owned, a party statement pointed out on Monday and it went on to call for "all questions and reserves" to be cleared up before going any further.
China, whose China General Nuclear Power Corporation (CGN) is committed to invest heavily, is also alarmed at the British government's deferment of a decision on the deal to build the controversial power station to the autumn.
Declaring that China-UK relations are at a "crucial historical juncture", Chinese ambassador to London Liu Xiaoming appealed to the British government to "come to a decision as soon as possible so that the project can proceed smoothly" in an article in Tuesday's Financial Times.
EDF unions slam boss
Three unions at the French energy company on Monday accused the government and EDF bosses of concealing the UK's intentions from the 28 July board meeting that gave the project the green light.
The British announcement came just hours after the vote, which approved the plan by 10 votes to seven.
That decision was "invalid", the CGT, CFE-CGC and FO unions declared in a joint statement, claiming that the government and chief executive Jean-Bernard Lévy were aware of the British government's intentions but did not tell the board.
President François Hollande had already given the green light to Lévy after meeting new Prime Minister Theresa May on 21 July.
May's predecessor, David Cameron, was an enthusiastic backer of the project but her advisers had expressed doubts at allowing Chinese investment in a sensitive operation, while some critics pointed to complications in the construction of other EPR reactors.
The French unions fear that the investment is too risky for EDF's future finances and was making the company too reliant on nuclear power.
Boss to sue union
Lévy last week said he would sue another union, Sud-Energie, for accusing him of lying to the board about Britain's intentions.
"The only element known on 28 July before the British government's announcement was the deferment of signing, which was initially envisaged for Friday 29 July," an EDF statement said, adding that this proposed date had never been made public so required no correction.
It threatened legal action against anyone "spreading such untruths".
Sud-Energie accused the company of overeacting to a leaflet for EDF employees that cited information from news media.
In a letter to board members dated 2 August, Lévy said he had been told that May "was asking for a bit more time" late on 27 July, according to the Reuters news agency.
On Friday a court rejected a bid by the works committee to have the board's decision annulled.