The new arrest in London

Counter-terrorism officers said on Thursday the dual UK-Iranian national had been arrested in London on suspicion of preparing terrorist acts.

• •The suspect is 27 years old
• •A second British man, 26, was questioned under caution
• •Searches were carried out at two properties

How the Sunday operation unfolded

The operation began in the early hours of Sunday, launched over a range of information that included an alert from a local farmer who had become suspicious of men she had seen near the base in three vans, police said. Those held were five British men in their 20s, arrested near RAF Fairford in western England on suspected terrorism and explosive offences. The Times newspaper reported that one of the men had called emergency services shortly before being arrested.

• •Detectives confirmed that, although petrol was found in the vans the men used, no improvised explosive device was present
• •The five men were later released without charge on police bail

Counter-terrorism police on the enquiry

Police have said they were considering whether a foreign state had been involved in the incident. Vicki Evans, Britain's Senior National Coordinator for Counter Terrorism Policing, said in a statement that officers were looking at all possible angles.

Iran summons British ambassador

Iran has dismissed accusations of its involvement as baseless, and on Thursday summoned the British ambassador in Tehran over the issue. The arrest relates to a suspected plot involving RAF Fairford, an airbase used by the US to target Iran. Prime Minister Andy Burnham said Britain was working closely with the US, whose bombers have in recent months launched missions against Iran from Fairford. The incident, which triggered a huge police response and raised alarm among Western allies, remains largely unexplained.