Energy Secretary Ed Davey will announce whether he is granting consent for French energy giant EDF to build a nuclear power plant at Hinkley Point C in Somerset.
The Government is expected to announce that it is giving the go-ahead to the first of a planned fleet of new nuclear reactors in the UK.
The plant's two nuclear reactors would be capable of producing 7% of the UK's electricity, enough to power five million homes, EDF said.
A final investment decision by the company to go ahead with construction also depends on a deal being negotiated with the Government on the "strike" price paid for electricity generated by the plant.
Under electricity market reforms, low-carbon power such as nuclear reactors and offshore wind farms will have long-term contracts with a guaranteed price for their electricity, to give investors certainty to invest in projects with high capital costs.
The Unite union's national officer Kevin Coyne said the expected go-ahead for the Hinkley Point C plant would be a significant milestone towards meeting the UK's low carbon energy needs and a massive boost for jobs.
He said: "The construction of Hinkley Point C will create thousands of skilled construction jobs for the next five years, and around 800 jobs in the operation of the power station over the next 60 years . We hope Hinkley Point C is just the first in a fleet of new nuclear power stations which would create jobs in construction for the next 20 years. Nuclear is a crucial part of a balanced energy policy, to stop lights going out."
But Greenpeace executive director John Sauven said Hinkley Point C failed the test on economic, consumer, environmental and arguably even legal grounds.
He said: "It will lock a generation of consumers into higher energy bills, via a strike price that's expected to be double the current price of electricity, and it will distort energy policy by displacing newer, cleaner, cheaper technologies.
"With companies now saying the price of offshore wind will drop so much it will be on par with nuclear by 2020, there is no rationale for allowing Hinkley C to proceed. Giving it the green light when there is no credible plan for dealing with the waste could also be in breach of the law."