Police are considering “every possible angle” including the use of proxies by a hostile state after five suspects were released on bail over the suspected terror plot near an RAF base.

Five British nationals were arrested on suspicion of explosives offences and terrorism on Sunday after three suspicious vans were found near RAF Fairford in Gloucestershire, which is used by the US.

It is understood that the material found was not viable explosives, but police have stressed that the investigation is continuing.

Speaking outside Scotland Yard on Monday, the head of UK Counter Terrorism Policing, Assistant Commissioner Laurence Taylor, said: “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.”

Acknowledging speculation over whether Iran was behind the plot, he said police are keeping an open mind, and one possibility could be proxies “either knowingly or unknowingly working on behalf of a foreign state”.

Iran, and other hostile states including Russia, has been known to use criminal proxies to carry out crimes on its behalf on UK soil.

Its London embassy earlier said it “categorically rejects and strongly condemns the recent unfounded and malicious speculations” of Iranian involvement.

Mr Taylor said: “I also know there is significant interest in whether the current geopolitical context has a bearing on this incident.

“At Counter Terrorism Policing, we are working within this context every single day and, at this stage, we are keeping an open mind in this investigation.

“We have, as you would expect, ensured that our protective security advice and posture remain commensurate to the threat.

“But be assured, we are considering this from every possible angle, including that this may be actively committed by proxies or individuals either knowingly or unknowingly working on behalf of a foreign state.

“To respond to incidents such as these, rapid mobilisation is necessary, as we cannot take the risk whilst assessing that threat.

“However, once we understand the threat more clearly, we seek to return affected communities to normality as quickly as possible.”

Police hope to be able to reduce the size of the cordon in place near RAF Fairford later on Monday, where dozens of homes have had to be evacuated as bomb disposal experts examined the vans.

Speaking at the Labour Party conference, Home Secretary Shabana Mahmood said evacuated residents will shortly be able to return home.

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.

Gloucestershire Constabulary was initially alerted to three suspicious vans travelling towards RAF Fairford at 12.45am 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 1.36am, and armed police arrived about 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?”

On Sunday, US President Donald Trump told reporters the suspects were looking to “do big damage”, and 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” adding that it was “too early to attribute background aims”.

Prime Minister Andy Burnham was holding a Cobra meeting on Monday “to make sure we have the full picture”, he told LBC.