Plans to build the first new nuclear plant in the U.K. in a generation were unexpectedly delayed after the government postponed making a final decision on the controversial £ 18 billion ($23.7 billion) project until the fall.

The news came late Thursday just hours after French state-controlled power utility Electricité de France SA's board voted to approve the project at Hinkley Point in Somerset, southwest England.

"The government will now consider carefully all the component parts of this project and make its decision in the early autumn," U.K. Business and Energy Secretary Greg Clark said.

EDF had expected to sign contracts to build two nuclear reactors on Friday.

The project has proved controversial in both France and the U.K., with some senior EDF officials and labor unions worried about its impact on the company's finances, while some politicians and environment groups in the U.K. consider it too expensive and risky.

The six labor-union representatives sitting on EDF's 18-member board have repeatedly opposed the project. Another board member, Gerard Magnin, resigned moments before the vote. According to Le Monde newspaper, he quit in opposition to the project. His departure follows the resignation earlier this year of the company's Chief Financial Officer Thomas Piquemal over the project.

Even though the U.K. government guaranteed EDF and its Chinese partner a price for the electricity generated by the reactors significantly above market levels, unions and several company officials have said the costs will saddle EDF with too much debt.

In the U.K., the guaranteed electricity price of £ 92.50 a megawatt hour for the 35 years has been criticized. The level is more than double the current wholesale power price in the U.K.

Concerns have also been raised that two other projects in Europe using the same reactor technology are years behind schedule and billions over budget.

The French government has backed the project and announced a cash injection of €3 billion ($3.3 billion) in EDF to help the company develop the project, which would lead to job creation in France and buoy beleaguered state-owned nuclear firm Areva SA, which manufactures nuclear reactors.

Write to Selina Williams at selina.williams@wsj.com and Inti Landauro at inti.landauro@wsj.com

(END) Dow Jones Newswires

July 29, 2016 05:15 ET (09:15 GMT)