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 was arrested in relation to offences under sections 15, 17 and 57 of the Terrorism Act, 2000, the force said.
The four members of the Beatles – so nicknamed because of their English accents – left the UK to fight in Syria and guarded, tortured and beheaded Western hostages. Davis has reportedly denied being a part of the group.
The group beheaded more than 27 hostages, according to US authorities, including American journalist James Foley and British aid worker David Haines.
His co-defendant, El Shafee Elsheikh, will be sentenced in August for his role in the plot.
Davis, who was known as Jihadi Paul, left the UK in 2013 to fight in Syria but was arrested in Turkey in 2015 and jailed in 2017 after being convicted of being a member of a terrorist organisation.
In 2014 his wife, Amal el-Wahabi, became the first person convicted of funding terrorism in Syria when she was jailed at the Old Bailey following a trial.
The pair met at a London mosque and became increasingly interested in Islam.
The court heard El-Wahabi tricked an old school friend into agreeing to take €20,000 (around £15,000 at the time) in cash to Turkey for her husband, who was from Hammersmith, west London.
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.”