In a surprise move, the British government said it would take time to consider the project, rather than giving it the green light immediately.
Business and Energy Secretary Greg Clark said nuclear energy was "an important part of the mix" for the U.K.'s energy supply.
"The government will now consider carefully all the component parts of this project and make its decision in the early autumn," said Clark, who took office this month in the new government of Prime Minister Theresa May.
EDF is majority-owned by the French government, which supports the project. The two reactors could ultimately produce 7 percent of British electricity and create 25,000 jobs, according to EDF.
One board member resigned Thursday to express disapproval of the project to build the reactors 165 miles (265 kms) west of London.
Gerard Magnin, who was among six people representing the French state on the EDF board, wrote in a letter seen by The AP: "I don't want to endorse any longer a strategy that I don't share."
EDF did not immediately comment on Magnin's resignation.
In March, one of EDF's senior vice presidents resigned. French media reported that Thomas Piquemal quit over concerns about the financing of the nuclear power plant.
John Sauven, executive director at Greenpeace, urged the British government not to sign the deal.
EDF "can barely hold itself together," he said in a statement. "Major figures are quitting in dissent, the company's employees are up in arms and a similar reactor being built in France is under investigation by the French nuclear regulator."
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