Three teenagers who were stopped from travelling to Syria from Turkey have been flown back to the UK and arrested, Scotland Yard was quoted by BBC news as saying.
Two boys aged 17 from north-west London and a man aged 19 were returned to the UK on Saturday night, the Met said.
They have been arrested on suspicion of preparing terrorist acts and are being held at a police station in London.
They were reported as missing on Friday and were detained after UK police alerted Turkish officials, it added.
Scotland Yard said counter-terrorism officers had initially been made aware that the two 17-year-old boys had gone missing and were believed to be travelling to Syria.
Further inquiries revealed they had traveled with a third man, UK police added.
"Officers alerted the Turkish authorities who were able to intercept all three males, preventing travel to Syria," a police spokesman added.
They were returned to the UK at about 23:10 GMT on Saturday and were arrested by counter-terrorism officers.