Counter-terror officers at Luton Airport have arrested a British woman suspected of preparing an act of terrorism and taken to a police station in south London.
Scotland Yard said that the 21-year-old was detained at 11:15am on Thursday after landing on a flight from Istanbul.
The development follows the arrest of a 19-year-old man and two 17-year-old boys, from north-west London, on Saturday on suspicion of preparing terrorist acts.
They were later bailed, as was an 18-year-old arrested by counter-terrorism officers from West Midlands Police on Monday.
The latest arrests came only weeks after three east London schoolgirls - Kadiza Sultana, Amira Abase and Shamima Begum - ran away to become so-called "jihadi brides".