The UK has signed its £18bn contract with France and China to build the Hinkley Point C nuclear power station, giving the final go-ahead for construction at the site in Somerset.

The deal was finalised at a low-key ceremony in London, just two months after Theresa May alarmed her French and Chinese counterparts by putting the entire project under review. EDF, the French nuclear contractor, and its Chinese partners had to cancel their previous plans for a signing ceremony at the last minute when the review was announced in July.

The project finally got approval this month, after Greg Clark, the business secretary, announced there would be some new restrictions on future investments in critical infrastructure if there were national security concerns.

Clark attended the signing ceremony on behalf of the UK, alongside Jean-Bernard Lévy, the chairman of EDF, and He Yu, chair of China General Nuclear.

The business secretary said it was a “crucial moment in the UK’s first new nuclear power station for a generation and follows new measures put in place by government to strengthen security and ownership”. The event was also attended by Jean-Marc Ayrault, the French foreign minister, and Nur Bekri, a senior official at the National Energy Administration of China.

He Yu said it signifies “CGN’s commitment to the UK as one of the world’s leading developers and operators of nuclear power. This flagship programme is a triple win for China, Britain, and France and is a culmination of years of cooperation between the three countries. CGN looks forward to providing UK consumers with safe, reliable and sustainable energy and maximising opportunities for UK suppliers and the UK workforce.”

Unions welcomed the development, saying thousands of skilled jobs would now be created, benefiting firms across the UK. It is expected to produce 7GW of electricity when fully operational, enough to power 6m homes and provide 7% of Britain’s electricity needs for 60 years.

However, Greenpeace said it was “no wonder the UK government has opted for a ‘champagne-free’ signing ceremony away from public view”.

The environmental group said: “With a stroke of the pen ministers are signing away billions of pounds of billpayers’ money to a project they know is plagued by legal, financial and technical problems. In the unlikely event Hinkley is working some time in the second half of the next decade, renewable energy will be much cheaper, yet British consumers will still be forced to pay over the odds for nuclear power. It’ll be like being locked into an expensive fixed-rate mortgage as interest rates plummet.”

Under the new controls, EDF will not be able to sell on its stake in Hinkley without permission. For future projects, the UK government will own a “special share” that means it will have a veto over owners if there are national security concerns.

In a sign the new requirements do not appear to be overly stringent, both EDF and CGN said they were delighted by the approval, which they claimed would let them proceed with Hinkley and their wider plans for nuclear construction in the UK in future.

The government published documents detailing the contractual obligations it has entered into, including a “value for money assessment” that allowed ministers to make the decision to proceed with the project. According to the documents, UK-based businesses would benefit from more than 60% of the cost of the project and 26,000 jobs and apprenticeships would be created during construction and after its opening.

Justin Bowden, the GMB union’s national secretary for energy, said: “With collective sighs of relief all round, it is fantastic news that the Hinkley deal is finally signed and work can start on this vital piece of UK infrastructure.

“The formal go-ahead for Hinkley is the first serious sign that UK plc is open for business post-Brexit and holding a secret ceremony did not do justice to such a historic event. Attention must now straight away shift to Bradwell B and Sizewell C.”