A 44-year-old man from Hertfordshire has been arrested at Gatwick Airport on suspicion of preparing terrorist acts.
Officers from the Metropolitan Police Counter Terrorism Command stopped the man after he disembarked from a flight from Iraq on Thursday.
A statement from the Metropolitan Police said: "As a result of enquiries during the stop a decision was made to arrest the man under section five of the Terrorism Act - suspicion of preparation of terrorist acts.
"The man was arrested under PACE and taken to a central London police station where he remains in custody."
It comes after a 31-year-old man arrested on Thursday in Norfolk on suspicion of fundraising for the purposes of terrorism and encouraging support for a banned terror group was bailed.
Scotland Yard said that arrest was related to suspected activities overseas. Two properties in east Norfolk and one in north London have also been searched.