Five men are being questioned by police after three “suspicious vehicles” were stopped travelling towards an RAF base used by US bombers to strike Iran.
Police swooped on the white vans near RAF Fairford in Gloucestershire in the early hours of Sunday following a tip off.
A huge cordon has been thrown around the area as scores of residents were evacuated.
In an update on Sunday afternoon, the Metropolitan Police said the five detained men had been further arrested on suspicion of the preparation of a terrorist act.
A Met statement said: “Counter Terrorism Policing can now confirm they are leading an investigation into an incident which took place overnight near RAF Fairford in Gloucestershire.
“At around 12.45am today (Sunday 27 September), police received a call relating to three suspicious vehicles which appeared to be travelling towards the air base.
“Armed officers responded and five men were arrested in the Whelford area on suspicion of committing offences under the Explosives Act. They were further arrested on suspicion of preparation of a terrorist act, contrary to section 5 of the Terrorism Act.”
Deputy Assistant Commissioner Vicki Evans, Counter Terrorism Policing Senior National Coordinator, said: “We are in the early stages of an investigation, and five men remain in custody. As you would expect we are working closely with our partners to understand the circumstances of the incident.
“We continue to work with Gloucestershire Police to keep the community safe as we work to gather evidence from the scene.
“A 400m cordon has been put in place around the vehicles while the Army’s Explosive Ordinance Disposal unit conduct examinations. As a precaution, around 85 households have been evacuated whilst these examinations take place.
The police operation near the village of Whelford was believed not to have been intelligence-led, meaning investigators had not known about any potential plot in advance.
A major incident has been declared in the Whelford, close to the base which has been used by the US Air Force to launch strikes on Iran.
A large emergency services presence was seen in the area, including a Hazardous Area Response Team.
Gloucestershire Police said: “A number of properties are currently being evacuated in the Whelford area following the declaration of a major incident.
“It follows the arrest of several men on suspicion of offences under the Explosives Act.
“The Army’s specialist explosive ordnance disposal team are currently examining a number of vehicles.
“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.”
The Ministry of Defence has been contacted for comment.
A spokesperson for the US Air Force said its personnel based at RAF Fairford “remain vigilant”.
They said: “We are aware of the current reporting, for questions regarding off-base incidents please contact the local authorities.
“For operational security reasons we will not discuss specific force protection measures.
“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.
“We constantly assess a variety of factors that determine what measures we institute or modify to protect our installations, our people and their families.”
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 (RIAT), though this year’s event in May was cancelled because of the military operations.
