Justin Bowden, national officer at the GMB union, called on Theresa May to approve the building of Hinkley Point C after French energy firm EDF insisted it was still behind the proposal.
EDF’s board approved the development earlier this year despite a fierce debate within the company over whether it was financially viable.
But, in a surprise move, Downing Street did not immediately follow suit and instead postponed a decision to the autumn.
It has been suggested the Prime Minister has concerns about the extent of Chinese involvement in the project and the implications for national security.
Mr Bowden said: “The faffing must stop now and the go-ahead on Hinkley must be given.
“It might be 80 degrees outside at the moment, but winter is on its way and GMB has repeatedly warned that if we have a cold one then the growing holes in our apology for an energy policy will result in power cuts.
“Everyone gets that, over time, sensibly sourced and UK-produced renewable energy sources makes sense.
“But wishful thinking doesn’t generate electricity and until there is a scientific breakthrough on carbon capture or solar storage, then nuclear and gas are the only reliable, and cost effective, shows in town.”
EDF chief executive Vincent de Rivaz today defended the project and the role of Chinese investment.
He said taxpayers would “pay nothing” towards the project and companies across Britain would benefit from the £18bn scheme.