- 13 updates
The World Nuclear Association, which promotes civil nuclear energy, reports that Sellafield has put the elevated radioactivity reading down to "variations in natural background".
Irish officials have been in contact with Britain over raised levels of radioactivity at the Sellafield nuclear reprocessing plant, the government has said.
Only around 112 miles (180km) separates the nuclear site from the coast of Ireland.
"Officials from the Department of the Environment have been in contact with their UK counterparts ... and will receive updates throughout the day," the Irish government said in a statement.
Professor Richard Wakeford, professor of epidemiology at the University of Manchester, has said that the level of radioactivity detected at Sellafield is not above that "encountered in everyday life".
The Nuclear Decommissioning Authority (NDA) has said it is too early to say whether there is a leak at the Sellafield site, but stressed that there was no danger to the public.
It added that it is still not clear where the radiation is coming from.
One Sellafield worker told the Press Association that an air sampler on a perimeter fence had detected a problem, which led to staff being told to stay away.
It is understood nothing has been detected inside the plant.
The World Nuclear Association, which promotes civil nuclear energy, reports that there are more than 20 radiation sensors at Sellafield, and only one of them is giving an elevated reading.
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.
The operator of the Sellafield nuclear site has said that the elevated levels of radioactivity pose "no risk to the general public or workforce".
A new statement says the decision to tell all but essential workers to stay home was made in order to "focus on investigation and avoid disruption on and off the site".
It added that the incident would not have prompted an evacuation if it had happened during the working day.