The suspects, who are all British nationals aged in their 20s and from London, were arrested over potential terror offences in Whelford, Gloucestershire, in the early hours of Sunday.

They were arrested on suspicion of explosives offences and later terrorism after a villager dialled 999 after she saw three vans and a group of masked men near the base.

Assistant Commissioner Laurence Taylor told reporters outside New Scotland Yard: “I can confirm that following extensive interviews and inquiries, the five men who were arrested are being released on police bail this afternoon.

“To be very clear, this does not mean that the investigation is over.

“They are subject to stringent conditions on their movement and contact with others.

“They absolutely remain under investigation as we explore multiple lines of inquiry.”

The five suspects are a 24-year-old from Camden, north London; three men aged 23, 24 and 25 from Westminster, central London, and one aged 24 from Willesden, north-west London.

Police are understood to be investigating whether any potential plot could be linked to Iran, which has used proxies to carry out crimes on British soil in the past.

Its London embassy said it “categorically rejects and strongly condemns the recent unfounded and malicious speculations” of Iranian involvement, while police asked to be given space to work amid “geopolitical questions” around motivation.

On Monday, Counter-Terrorism Policing (CTP) said that multiple new lines of inquiry have emerged after investigators were working at the scene overnight.

CTP Senior National Coordinator, Deputy Assistant Commissioner Vicki Evans said: “This is a hugely complex and large-scale operation, involving local and national policing, the MoD, and local agencies, who are working together to progress the investigation and support for the community.

“I’d also like to thank the emergency services for their response, including the armed officers who first attended the scene, not knowing what they may face.

“I am very aware of the levels of speculation about the motivation behind this incident, and the geopolitical questions that are being raised. However, I ask that at this time, Counter Terrorism Policing are given the space to work carefully and clearly.”

Gloucestershire Constabulary were initially alerted to three suspicious vans travelling towards RAF Fairford at 12.45 am on Sunday, and it is unclear what action was taken in the hour that followed.

The local farmer who spotted the vans and a group of masked men initially called the Ministry of Defence police, but there was no answer, so called 999 at 1.36 am, and armed police arrived around 10 minutes later.

She said the police had “dropped the ball”, telling journalists: “Who knew that a farmer would have to roll their sleeves up and report the activity?”

A pensioner living nearby described an apparent increase in Fairford’s security in recent weeks, telling PA that heavy tractor-like vehicles had been parked and concrete walls laid behind the gates.

Aerial footage showed a bomb disposal robot spending hours inspecting three white vans, two of which had open rear doors. Large black barrels appeared to be scattered around.

On Sunday, US President Donald Trump told reporters the suspects were looking to “do big damage”, while on Monday, Defence Secretary Wes Streeting said the UK must not be “cowed by sabre-rattling from Iran” or allow the country to “dictate our foreign policy,” while stressing it was “too early to attribute background aims”.