Counter-terrorism police have arrested four British men at Gatwick Airport.
Officers detained the men, in their 30s, after they were refused entry to Turkey last week, Scotland Yard said.
They were arrested under the Civil Aviation Act 1982 after being flown back to the UK on Saturday, a Metropolitan Police spokesman said.
He said: "On Saturday May 2, officers from the Met's counter-terrorism command arrested four men aged 36, 38, 37 and 32.
"They were arrested for an offence under the Civil Aviation Act 1982.
"The men were taken to south London police stations and bailed to return to a date in mid-June."
The men were stopped at Antalya airport in Turkey on April 29, police said. It is not known why the Turkish authorities refused them entry.