Energy giant EDF is set to make its long-awaited final investment decision on the planned nuclear power station at Hinkley Point, ending doubts over the massive £18 billion project.
The French firm's board is meeting in Paris on Thursday and is expected to give the go-ahead for the first nuclear power station to be built in the UK for a generation.
UK unions will warmly welcome the much-delayed decis ion, saying workers were "raring to go" - with 25,000 jobs set to be created.
But environmental groups such as Greenpeace will criticise the go-ahead, calling for investment in home grown renewable energy like offshore wind.
Fresh criticism is also expected on the Government's promise to pay EDF £92.50 for each megawatt hour of energy it generates.
Hinkley Point C (HPC) will provide 7% of the UK's electricity over its estimated lifetime of 60 years and is scheduled to begin generating power in 2025, several years later than planned.
The main reason for the delay has been worries over the financing of the project by EDF, which is 85% owned by the French government, with French trade unions warning it could ruin the company's finances.
China General Nuclear Power Corporation (CGN) is set to be confirmed as taking a 33.5% stake in the project.
Ahead of the decision, EDF said: "HPC is a unique asset for French industry as it would benefit the whole of the nuclear industry and support employment in major companies and smaller enterprises in the sector.
"This project has been the subject since 2013 of a significant sharing of information with employees and their representatives, illustrating the commitment of the company to quality social dialogue."
Unite national officer for energy Kevin Coyne said: "We urge the EDF board to give the financial go-ahead on a project which will generate thousands of decent skilled jobs and help meet the energy needs of the UK for generations to come.
"The cost of not doing so could result in the lights going out in Britain and the West Country missing out on the much needed economic boost which this major infrastructure project would bring.
"Unite has already done a lot of preparation in reaching agreements with contractors over pay and skills, as well as safeguards against blacklisting on the site. Workers are shovel ready and raring to go, all they need is the green light from the EDF board."
Garry Graham, deputy general secretary of the Prospect union, said: " Energy margins for the UK continue to decline and we need to take the practical steps which ensure that we keep the lights on and transition to low carbon generation.
"A final investment decision will be good for jobs, good for consumers, good for the environment and good for the economy."
John Sauven, executive director of Greenpeace, said: " Every time EDF has tried to build a reactor like Hinkley, it has failed. There isn't a shred of evidence that Hinkley can be built on time or on budget, and if it hits the same problems as its predecessors, it can't be relied on to keep the lights on in the UK.
"The UK Government doesn't have to sign the contract with the French and Chinese state- owned nuclear companies.
"We need to invest in reliable home grown renewable energy like offshore wind which is powering other northern European countries more cheaply than Hinkley, even taking into account the back-up cost when the wind doesn't blow."
Justin Bowden, national officer of the GMB union, said: "This decision is an acid test for Theresa May's claims that the UK is open for business after Brexit with tens of thousands of jobs on the line. 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 that keeps the lights on, our homes heated, and the economy functioning.
The final investment decision on Hinkley Point C is fundamental to a balanced energy policy - something the country desperately needs and which the Government has patently failed to provide."