Counter-terror detectives have raided a string of houses after five men were arrested close to the Sellafield nuclear site in Cumbria.
Sky sources said it is understood the men being held were Bangladeshis and were taking photographs of the plant.
Four houses were raided in east London following the arrests at about 4.30pm on Monday.
A small box containing an unknown substance was removed from one of the properties for testing.
The men were detained under section 41 of the Terrorism Act after a stop check on a vehicle.
The five, all in their 20s and living in London, were held overnight at a police station in Carlisle.
They were then taken to Manchester, where the investigation is being led by counter-terrorism officers.
Highly dangerous nuclear materials are contained at Sellafield
Sky's home affairs correspondent Mark White said: "From what I understand there was nothing found on them that would give immediate cause for concern that there was any potential attack."
A spokesman for Greater Manchester Police said the investigation was in its early stages and no further information would be released yet.
He added: "At this stage we are not aware of any connection to recent events in Pakistan."
The Government has urged the public to remain vigilant.
According to reports last year, a counter-terrorism review of Britain's nuclear power plants was carried out after fears arose over safety at Sellafield.
The Sellafield site - responsible for decommissioning and reprocessing nuclear waste, and fuel manufacturing - is heavily protected by private security and officers from the Civil Nuclear Constabulary, some of whom are armed.
Section 41 of the Terrorism Act 2000 allows a police officer to arrest any person who is "reasonably
suspected" of being a terrorist.