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 is being held under section 5 of the Terrorism Act - which prohibits anyone engaging in conduct in preparation for acts of terrorism - and is being held at a police station in Kent.
“A 26-year-old man from Kent has today been arrested by counter-terrorism policing South-East officers on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism under section 41 of the Terrorism Act 2000,” said a police spokesperson.
“He has been detained at a police station in the south-east for questioning. A residential address in Kent is being searched in connection with the ongoing investigation.”