* Low-key ceremony contrasts with previous fanfare
* PM May ordered review of project before giving go-ahead
* Critics say project too costly (Adds reaction, background)
By Kate Holton and Karolin Schaps
LONDON, Sept 29 (Reuters) - A $24-billion deal to build Britain's first new nuclear power station in decades was signed behind closed doors in London on Thursday in a private ceremony that underlined Prime Minister Theresa May's cautious approach to the Franco-Chinese project.
Journalists were not invited to the event but the government said the contract had been formally signed by Britain's Business Secretary Greg Clark, French Foreign Minister Jean-Marc Ayrault and China's National Energy Administration Director Nur Bekri.
Hinkley Point C will now be built in southwest England by France's EDF with $8 billion of cash from China.
"Britain needs to upgrade its supplies of energy, and we have always been clear that nuclear power stations like Hinkley play an important part in ensuring our future low-carbon energy security," Clark said in a brief statement.
The no-frills event, lacking in pomp or publicity, contrasted sharply with the approach of May's predecessor David Cameron, who secured China's involvement in the project during a lavish state visit laid on for President Xi Jinping last year.
The signing ceremony also followed months of uncertainty.
The French state-controlled utility EDF had to overcome a bruising boardroom battle in order to approve the deal in July, only to see Britain's new prime minister put it on hold just hours later, stunning Paris and Beijing and scuppering a planned large signing ceremony due to be held at the plant.
A former colleague said May had been concerned by the security implications of the planned Chinese investment.
Hinkley was finally given the green light earlier this month after the government included the proviso that it would be able to block the sale of EDF's controlling stake. The government also said it would take a more cautious approach in future over foreign investment in big infrastructure projects.
The EDF board confirmed its approval of the project again in a meeting on Wednesday.
"It's a good deal," France's Ayrault told reporters earlier in the day. "I know it has raised questions, particularly in France, but it represents an opportunity for the future of an entire industry."
The deal - in the works for more than a decade and the first in a series of new nuclear projects in Britain - is part of a recovery of the global nuclear power industry following a slump caused by the 2011 Fukushima disaster in Japan.
The first of the two new reactors at Hinkley Point is scheduled to be running by the middle of the next decade. Hinkley Point will provide around 7 percent of Britain's electricity, helping to fill a supply gap as the country's coal plants are set to close by 2025.
Critics have said the project is too expensive and does not reflect falling energy prices since the deal was drawn up, or anticipated declines in the costs of rival clean technologies like wind and solar power.
At 92.5 pounds per megawatt hour, Britain has agreed to pay roughly double the current market price for electricity for 35 years.
"It's no wonder the UK government has opted for a 'champagne-free' signing ceremony away from public view," Greenpeace UK executive director John Sauven said.
According to documents released by the department of business, 64 percent of materials and services used on the plant will be British, while EDF said it would not call on a 2 billion pound loan offered by the British government.
EDF said the project was of strategic importance to the French firm and the nuclear industry in general.
"Hinkley Point C will kickstart Britain's nuclear revival," EDF Energy CEO Vincent de Rivaz said. "It has overcome obstacles and challenges which will benefit our next nuclear projects in Britain."
China also plans to make a number of investments in British nuclear power, including the building and operating of a new station with EDF at Bradwell-on-Sea, southeast England. Bradwell would be a Chinese-led project, using Chinese reactor technology. (Additional reporting by Helen Reid in London and Geert de Clercq in Paris; editing by Stephen Addison and Gareth Jones)