The unnamed teenagers were detained on Friday in the Turkish city of Istanbul.
* Three males arrested after return to Britain, police say
* Had been detained in Istanbul before being deported
* Thought to be trying to join Islamic State in Syria
Reuters/Ankara
Three male British teenagers suspected of planning to join Islamic State militants in Syria have been arrested by London police after being deported from Turkey, officials said on Sunday.
The three, who have not been named, were detained on Friday in the Turkish city of Istanbul, Turkish sources told Reuters, after a tip-off from British authorities that two of them were travelling to Turkey via Spain.
London police said they had been made aware on Friday that two 17-year-old boys from the city had gone missing and were thought to be travelling to Syria. Further inquiries revealed they had travelled with a 19-year-old male, police said.
They were then flown back to Britain late on Saturday, when they were arrested "on suspicion of preparation of terrorist acts", the Metropolitan Police statement said. Turkish authorities confirmed that they had been deported.
They are being held at a London police station, the statement said.
Their arrest comes after three London schoolgirls entered Turkey last month and are thought to have joined Islamic State in Syria.
Security services estimate some 600 Britons have gone to Syria or Iraq to join militant groups, including the man known as "Jihadi John" who has appeared in several Islamic State beheading videos.
Hundreds of other Europeans have also joined the fight.
Their involvement has raised fears about the possibility of attacks at home if they return trained and further radicalised.
Turkey meanwhile has faced criticism for not better controlling its southeastern borders, and has accused European countries of failing to prevent would-be jihadists from travelling in the first place.
On Thursday, the Turkish foreign minister said the missing girls who had earlier travelled to Syria were helped to cross the border by a spy working for one of the countries in the US-led coalition against the militants.
Islamic State controls swathes of territory in Syria and Iraq where it has declared an Islamic caliphate.
