A 42-year-old man has been arrested on suspicion of terror offences as he entered the UK from France on a coach.
The man was detained at Folkestone, Kent, at around 12.50am on Tuesday, police said.
The arrest relates to the Syrian conflict, the Metropolitan Police added.
The detained man was held under section 5 of the Terrorism Act – which prohibits anyone engaging in conduct in preparation for acts of terrorism – and was questioned at a police station in Kent.
He was released under investigation on Tuesday evening.