French utility giant EDF is expected to take the long-delayed final investment decision on its Hinkley Point C nuclear power station in Somerset.
EDF board members are meeting in Paris today, where they're expected to greenlight the controversial project.
UK unions have thrown their weight behind the project, which will create tens of thousands of jobs and boost the local economy.
Phil Whitehurst, GMB National Officer engineering construction, said: "This decision is an acid test for Theresa May’s claims that the UK is open for business after Brexit ... we've heard lots of warm ministerial words about the UK’s commitment to nuclear power, but these will remain hollow unless they are backed by a decision to invest."
Prospect deputy general secretary, Garry Graham, added: "A positive FID will be a shot in the arm for the UK nuclear and construction supply chain and send a clear message that the UK is open for business."
But the decision has been criticised by environmental groups such as Greenpeace, who want more investment in renewable energy sources such as solar power and wind turbines.
It comes days after the state-owned firm's shareholders approved plans to issue new shares to raise 4bn euros (£3.4bn) towards the cost of Hinkley.
Hinkley has led to conflict within the ranks of EDF, with some unions warning the huge costs could sink EDF.
The company's chief financial officer, Thomas Piquemal, unexpectedly resigned earlier this year over EDF's ability to finance the project.
The UK must replace about 20 per cent of its ageing nuclear and coal power plants during the next 10 years, and Hinkley Point power is integral to keeping the lights on.