A man was arrested on suspicion of terrorism at Luton Airport this week while trying to travel to Turkey.
Scotland Yard said the 22-year-old suspect was held on Wednesday afternoon by officers from Bedfordshire Police before he got on the flight to Istanbul.
He was detained on suspicion of being involved in the preparation of acts to commit terrorism and taken to a police station in south London where he is still being questioned.
Would-be jihadis who are trying to travel to Syria often fly to Turkey and then travel overland to the war-torn Middle Eastern state.
Hundreds of Britons are believed to have gone there to learn terrorist tradecraft and fight with extremist jihadist groups.
A statement from the Metropolitan Police said: "Officers from the Metropolitan Police Service's Counter Terrorism Command (SO15) are investigating following the arrest of a 22-year-old man at Luton Airport.
"At approximately 4.30pm on Wednesday July 9 the man was stopped under schedule 7 at Luton Airport by officers from Bedfordshire Police before boarding a flight to Istanbul, Turkey.
"He was subsequently arrested by the officers on suspicion of being involved in the preparation of acts to commit terrorism, contrary to Section 5 of the Terrorism Act 2006.
"He was taken to a South London police station where he remains in custody."
A search has been carried out at an address in west London as part of the investigation.
On Wednesday, three men were arrested on suspicion of terrorism offences linked to the country.
The men, from Berkshire and Buckinghamshire, were bailed by police until dates in August.
