British counter-terrorism police investigating a suspected plot involving an air base used by the US to target Iran said on Thursday they had arrested a dual UK-Iranian national on suspicion of preparing terrorist acts.

Axar.az informs, citing Reuters, in an incident which triggered a huge police response, raised alarm among Western allies, and remains largely unexplained, five British men in their 20s were arrested on Sunday on suspected terrorism and explosive offences near RAF Fairford in western England.

The men were later released without charge on police bail. But counter-terrorism police said they were considering whether a foreign state had been involved in the incident and Prime Minister Andy Burnham said on Wednesday it might have had an Iranian link.

Iran has said accusations it was involved were baseless and on Thursday summoned the British ambassador in Tehran over the issue.

In the latest development, police said they had arrested the 27-year-old dual national in London while a second British man, 26, had also been questioned under caution and searches carried out at two properties.

“The investigation into circumstances surrounding events in Gloucestershire is hugely complex, and our specialist teams are interrogating multiple lines of enquiry,” Britain's Senior National Coordinator for Counter Terrorism Policing, Vicki Evans, said in a statement.

“As we’ve made clear, we’re looking at all possible angles – including possible foreign state involvement.”