The 27-year-old man was arrested in the Borough of Westminster, London, on suspicion of preparation of terrorist acts, British police confirmed. A second British man, 26, was also questioned under caution.
The arrests represent the latest developments in a sprawling investigation that began on Sunday when five British men in their 20s were arrested near RAF Fairford in Gloucestershire on suspicion of terrorism and explosive offences. Those men were later released without charge on police bail under stringent conditions.
Police launched a major operation after a local farmer became suspicious of the men, who were seen near the base in three vans. The Times newspaper reported that one of the men had also called emergency services shortly before being arrested.
Detectives have confirmed that although petrol was found in the vans, no improvised explosive device was present.
British Senior National Coordinator for Counter Terrorism Policing Vicki Evans described the investigation as "hugely complex" and said police were "looking at all possible angles – including possible foreign state involvement."
Prime Minister Andy Burnham has said the incident may have involved Iranian involvement, and Britain is working closely with the United States. US President Donald Trump said he would not have released the men, and Secretary of State Marco Rubio pointed to the "hands of a foreign actor" in the incident.
Amid heightened tensions, the Iranian government summoned the British Ambassador in Tehran on Thursday afternoon.