Villagers have been evacuated from their homes near RAF Fairford in Gloucestershire.
Counter-terrorism police are leading the investigation that saw several men arrested for explosive offences near an RAF base, the Press Association understands.
The police operation near the village of Whelford in Gloucestershire was not intelligence-led, meaning investigators had not known about any potential plot in advance.
Villagers have been evacuated from their homes near RAF Fairford, which is used by the United States Air Force (USAF), and the Army’s explosives experts are examining vehicles, police said.
A large emergency services presence was seen in the area, including a Hazardous Area Response Team.
Prime Minister Andy Burnham is receiving updates on the situation.
Gloucestershire Police said: “A number of properties are currently being evacuated in the Whelford area following the declaration of a major incident.”
