Counter-terrorism police are leading the investigation that saw several men arrested on suspicion of 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.”
The arrests come amid heightened concerns around the threat from hostile states to the UK, with Russia and Iran among those that have used proxies to commit crimes on their behalf on British soil.
In March, officials said the number of national security cases involving activity by hostile states including Iran increased 50% in the six months to December 2025.
Investigators will be working to analyse any devices seized from those arrested to establish details of any plot and any potential motivation.
Gloucestershire Police said in a statement that several men had been arrested on suspicion of offences under the Explosives Act.
“The Army’s specialist explosive ordnance disposal team are currently examining a number of vehicles,” the force said.
“The local residents are being evacuated to a nearby leisure centre.
“A cordon is currently in place but we’d like to reassure people that we believe this incident is contained.
“We are working closely with our partners and are enacting the well-rehearsed plans we have in place.”
A spokesperson for the USAF said its personnel based at RAF Fairford “remain vigilant” but “for operational security reasons we will not discuss specific force protection measures”.
They said: “The 501st Combat Support Wing and our fellow UK-based Wings remain vigilant and will take appropriate actions to ensure the safety and security of our US service members, civilians, contractors, and their families.”
The 501st is “the only combat support wing in the USAF” and operates from 10 bases including Fairford, according to its website.
“We constantly assess a variety of factors that determine what measures we institute or modify to protect our installations, our people and their families”, the spokesperson added.
RAF Lakenheath in Suffolk is on Charlie alert, the second-highest threat level that can be issued by the US Department of Defence.
US airbase Lakenheath confirmed its current threat level to the Press Association.
This comes after reports that RAF Fairford has been raised to the highest threat level, Delta.
Force Protection Condition (FPCon) Charlie “applies when an incident occurs or intelligence is received indicating some form of terrorist action targeting personnel or facilities is likely”, according to a US government website.
Delta “applies in the immediate area where a terrorist attack has occurred or when intelligence has been received that terrorist action against a specific location or person is imminent”.
Defence Secretary Wes Streeting confirmed the incident near RAF Fairford is “being dealt with” and thanked the emergency teams who acted “so swiftly”.
He added: “It is exactly why our emergency response teams and Armed Forces are on standby 24/7.”
A Government spokesperson said: “We are grateful to the emergency services, who have acted quickly to detain several suspects and put measures in place to keep the community safe.
“It would be inappropriate to comment further whilst investigations are ongoing.”
South Cotswolds Liberal Democrat MP Roz Savage described the situation as “worrying”.
She said on X: “I have contacted the police for further updates and I’ve been told that it is believed the incident is contained.
“I would like to thank them for the work they are currently doing.”
The Ministry of Defence has been contacted for comment.
In March this year, after then prime minister Sir Keir Starmer granted permission for “defensive” US action against Iranian missile sites from UK bases, four American bombers landed at RAF Fairford and the US started using British bases for “specific defensive operations”.
The airbase also hosts the annual Royal International Air Tattoo, though this year’s event in May was cancelled because of the military operations.
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