Police have arrested a man at Heathrow Airport on suspicion of committing acts of terrorism after he arrived on a flight from Bahrain.
The 24-year-old was detained by officers from the Metropolitan Police's Counter Terrorism Command on Monday morning.
He was taken to a south London police station where he is being questioned over alleged terrorism.
A police spokesman said: "The man was arrested after initially being stopped by immigration officials at 8am.
"Today's arrest forms part of an investigation into travel to Syria in support of alleged terrorist activity."
Another man, Shajul Islam, 26, is facing trial after he was arrested in the same inquiry.
The trainee NHS doctor is charged with holding British photographer John Cantlie and a Dutch journalist captive .
Islam, who studied at St Bartholomew's and a University of London hospital, is accused of being part of an extremist group that held Mr Cantlie, who worked for various newspapers including the Sunday Times, and Jeroen Oerlemans for around a week in the war-torn state.
Islam, from Stratford, east London, was arrested at Heathrow Airport last month as he returned to the UK with his wife and one-year-old daughter on a flight from Egypt.
Mr Cantlie and Mr Oerlemans were kept handcuffed and blindfolded after being captured as they travelled into Syria to cover the conflict there.
It is alleged that Islam, with others, unlawfully and injuriously imprisoned Mr Cantlie and Mr Oerlemans against their will, between July 17 and 26.
Islam is due to appear at the Old Bailey on April 12 for a plea and case management hearing. A six-week trial is due to begin on June 5.