Non-essential workers at the Sellafield nuclear reprocessing plant have been told to stay at home on Friday in light of the London bombings.
Only essential staff are being allowed into the plant.
The decision to increase the level of alert at all nuclear sites across the UK was made by the government's Office of Civil Nuclear Safety.
Sellafield bosses say they have acted to restrict access and limit the number of personnel and vehicles on the site.
Spokesman Pete Lutwitch said that the move was a precautionary measure.
He said: "We've got very well-practiced arrangement when we increase the level of alert.
"Only those essential staff will be in place. These include our medical teams, safety teams and site management.
"Our prime responsibility that everyone stays safe is right at the top of our agenda.
"This is a decision made by the government. It is a prudent and conservative response to what happened in London yesterday."
Mr Lutwitch dismissed claims that the emergency measures were in response to a bomb threat at Sellafield.