The Sellafield nuclear site is being operated with a reduced number of staff following the detection overnight of elevated levels of radioactivity.

Non-essential staff are being told not to come to work, although this is described as a precautionary measure, and in a statement the site's operator said there was "no risk to the general public or workforce". It is unclear how long the plant will operate with reduced staffing but a few hundred workers are understood to be on the site.

The statement from Sellafield Ltd stated: "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.

"We have taken this decision to focus on investigation and avoid disruption on and off the site (such as traffic disruption in the west Cumbria region)."

"If this had happened during the working day we would just allow people to go home as normal. There is no risk to the general public or workforce."

The government's safety watchdog, the Office for Nuclear Regulation, said: "We are satisfied that Sellafield continues to take appropriate action in response to the enhanced activity levels. The levels are not increasing and we are satisfied that Sellafield is making progress with determining the source of the activity. We expect to have further information around midday."

The Department of Energy and Climate Change (Decc) confirmed the elevated radioactivity but said they were "well below levels of concern". The plant was still operating and not offline, she said, but all non-essential staff had been asked to leave the site.

Gary Smith, the national secretary for energy at the GMB union, said the majority of Sellafield's workforce had been told not to turn up for work on Friday morning. The site employs more than 10,000 people.

The unusual radiation reading at the perimeter fence had been detected at 2am, he said. But his members at Sellafield told him that they were not aware of any problems at any of the 1,000 nuclear facilities on the site. "There's a possibility that this is a faulty air sampler," he said.

Gill Wood, the national secretary of another union that represents 5,000 Sellafield staff, Prospect, said: "Non-essential staff have been advised to stay at home today while the relevant specialist team investigates. The company's decision to partially close some areas at the site is a precautionary and measured decision and a safety measure that is recognised worldwide."

A local newspaper, News and Star, reported that around 8,000 workers were affected. A Decc spokeswoman was unable to confirm that figure.

Richard Wakeford, professor of epidemiology at the University of Manchester, said that asking non-essential workers to stay home was "a prudent precaution until the cause is known and the situation rectified".

He added: "There are no operating nuclear reactors at Sellafield, so short-lived radionuclides such as iodine-131 are not present there to any significant extent – therefore no stable iodine tablets should be taken as there is no need.

"If long-lived radionuclides, such as caesium-137, are responsible, then it will be important to find out what they are so that their source can be identified."

The local anti-nuclear group, Cumbrians Opposed to a Radioactive Environment (Core), said Sellafield was putting out "mixed messages" about the radiation that were "doing little to instil public confidence".

The group claimed that if there were elevated levels of radioactivity at the perimeter fence, there were probably high levels outside the site.

"The warning to non-essential staff to stay home is an action by the company well above and beyond normal action," said a spokesman, Martin Forwood. "We understand that it may take some time for the company to track down the source of the raised activity but the sooner they [Sellafield] level with the public by providing a more coherent explanation of what's going on the better."

The 50-strong group of Nuclear Free Local Authorities (NFLA) said it was "extremely concerned" about elevated levels of radioactivity at Sellafield. "The decision to partially close the Sellafield site and reduce its normal operations for the day due to elevated levels of radioactivity has to be of concern for all worried about this ageing facility," said the NFLA chair, the Manchester councillor Mark Hackett.

"I visited Sellafield a few months ago and was personally concerned about how decrepit some of the oldest parts of the site looked some 25 years after my first visit. I urge a full and independent inquiry into the source of this radiation leak and, while I welcome initiating prompt emergency procedures on the site, this is another example of the ongoing danger of dealing with such sensitive and potentially harmful materials."

The management at the plant has been criticised by parliament's public accounts committee (Pac) over the past year. In a report in February last year MPs said Nuclear Management Partners (NMP), which manages Sellafield, had failed to stem rising costs and delays in dealing with waste and the decommissioning of facilities.

"It is unclear how long it will take to deal with hazardous radioactive waste at Sellafield or how much it will cost the taxpayer. Of the 14 current major projects, 12 were behind schedule in the last year and five of those were over budget," Margaret Hodge, the Pac chair, said at the time.

MPs on the committee said they suspected that the NDA, a public sector body established to oversee the safe dismantling of the UK's old nuclear power stations and deal with waste, did not have a tight enough rein on NMP – a consortium made up of Amec of Britain, Areva of France and the US firm URS – to properly control costs.

The timing of the radioactivity warning may prove embarrassing for David Cameron, who is meeting the French president, François Hollande, to discuss joint efforts on nuclear power at a pub lunch in Oxfordshire. The UK and France signed an agreement in 2012 to work together on civil nuclear energy. The largely French state-owned EDF is building the Britain's first new nuclear power plant in decades at Hinkley Point in Somerset.