On his arrival to the UK he was arrested by the Metropolitan Police’s Counter Terrorism Command. He was taken to a south London police station and remains in custody.
Davis is said to have denied he was part of the Beatles cell - so nicknamed because of their English accents - which tortured and beheaded western hostages in Syria.
Cell ringleader Mohammed Emwazi, widely known as Jihadi John, was killed in a drone strike in 2015. Londoner Alexanda Kotey was jailed in the US in April for his part in the torture and murder of American hostages.
His co-defendant, El Shafee Elsheikh, will be sentenced in August for his role in the plot.
A statement from the Met Police on Wednesday evening said: “Officers from the Met’s Counter Terrorism Command have today, Wednesday 10 August, arrested a man at Luton airport.
“The 38-year-old man was arrested this evening after he arrived into the UK on a flight from Turkey.
“He was arrested in relation to offences under sections 15, 17 and 57 of the Terrorism Act, 2000 and was taken to a south London police station, where he currently remains in police custody.”