French Investment into a huge new nuclear power plant at Hinkley Point has been confirmed, according to reports. The proposed plant is set to become the first new nuclear plant to be built in the UK in 20 years at an existing site in Somerset.

Despite the green light from EDF, the government postponed a final decision until the early autumn. Business and Energy Secretary Greg Clark said: "The UK needs a reliable and secure energy supply and the government believes that nuclear energy is an important part of the mix.

"The government will now consider carefully all the component parts of this project and make its decision in the early autumn," added Clark.

Supporters of the deal – with French firm EDF – say Hinkley Point C will power 5.8 million homes and create 25,000 jobs. Sky News reported that the Paris-based board voted 10-7 in favour of investing in the site aimed at replacing Hinkley Point B, which is due to be decommissioned in the next 10 years.

The next step in the process is for legally-binding contracts to be signed so that construction work can begin – with EDF hoping to have more than 2,500 workers on site by 2017. Earlier on Thursday (28 July), EDF board member Gerard Magnin stepped down, saying the multi-billion pound project was financially risky.

Opponents warn of possible damage to the environment. Other concerns include the level of foreign investment, with one third of the £18bn (€21.5bn; $24bn) needed being provided by Chinese investors, who are also planning to invest in another decommissioning site at Bradwell-on-Sea in Essex.

Greenpeace executive director John Sauven, said the decision was "a bitter pill to swallow for hard-up people who have been told that the government is trying to keep bills down while dealing with energy security and lowering carbon emissions."

He added: "Today's decision doesn't prove the UK is open for business post Brexit – it just shows the Hinkley deal became too big to fail in the eyes of British and French politicians."

The project is scheduled for completion in 2025. The reactors are among the biggest in the world and will provide up to 7% of Britain's electricity needs when they become operational.

The UK government has agreed a strike price – a guaranteed price for the electricity generated by Hinkley Point – of £92.50 per megawatt hour for 35 years. This is more than twice the cost of existing wholesale electricity prices.

Since the government agreed the deal, the cost of wholesale energy has dropped, meaning the state would have to fill in the difference. The UK's National Audit Office estimated that future top-up payments for the plant would rise from £6.1bn to £29.7bn over the length of the contract.

Chancellor Philip Hammond said recently that the scheme was "still worth the cost", suggesting that customers' bills would rise once the plant was up and running.