The Sellafield nuclear site is being operated at "reduced manning levels" following "the detection of elevated levels of radioactivity".
Operator Sellafield Ltd said in a statement: " As a result of a conservative and prudent decision, the Sellafield site is operating normally but with reduced manning levels today.
"This follows the detection of elevated levels of radioactivity at one of the on-site radiation monitors at the north end of the site.
"Essential workers only are being asked to report for work.
"Levels of radioactivity detected are above naturally occurring radiation but well below that which would call for any actions to be taken by the workforce on or off the site.
"The site is at normal status and employees and operational plants are continuing to operate as investigations continue. All our facilities have positively confirmed there are no abnormal conditions and are operating normally."
One worker said that only safety and essential staff were in work today.
He said other employees had been told not to turn up for work, but had not been given any details of what had happened at Sellafield.
He estimated that thousands of workers were affected.