A 21-year-old man has been arrested at Gatwick Airport on suspicion of getting terrorist training abroad.
Scotland Yard said the suspect was held after he got off a flight from Istanbul at around 2pm today and is being questioned at a police station in west London.
He was held on suspicion of an offence under Section 6 of the Terrorism Act 2006. Police said they could not give any further detail about the man's arrest at this stage.
Passing through Turkey is a well-established route for people travelling to and from Syria.
Counter-terrorism investigators have expressed concern about aspiring British jihadis travelling from the UK to Syria to learn how to fire guns and make bombs.
It emerged last month that 250 UK-based extremists who went to train and fight in Syria have returned to Britain.
In January alone, 16 people were arrested on suspicion of terrorism offences related to Syria compared with 24 arrests in the whole of last year.
There were also reports that the first UK jihadi had staged a suicide attack in Syria.
Abdul Waheed Majeed, 41, was suspected of being responsible for driving a lorry to a jail in Aleppo and detonating a bomb. He is among an estimated 20 Britons who have been killed in the fighting in the war-torn state.
He was later bailed to return to a London police station tomorrow, Scotland Yard said.