The Moorside nuclear power station was scheduled to begin producing electricity from one of three reactors in 2024.
But on Wednesday NuGen chief executive Tom Sampson said the station will not be up and running until after 2025, with a new target set to 2030.
The development has been hindered by a series of setbacks including the liquidation of Westinghouse — the US arm of Japanese firm Toshiba — which was due to provide the station’s reactors.
As a result French investor Engie pulled out of the project. NuGen is hoping that state-backed China General Nuclear will take over.
Calling for the government to intervene in the development, GMB senior organiser Chris Jukes said: “This delay is far from a surprise and finally confirms what GMB has been saying for months.
“We welcome the honesty from NuGen and ask why the government is so silent and failing to show leadership on Moorside.
“Britain needs this vital new infrastructure, and the reliable zero-carbon electricity it will produce, and it is the government’s responsibility to make sure it is built and in a timely manner.”
Moorside is sited next to the Sellafield nuclear reprocessing plant.
In September 2014, Cumbria residents launched the campaign Radiation Free Lakeland in a bid to protect the idyllic countryside around Moorside from nuclear waste.