The militants, dubbed "The Beatles" by their hostages because of their British accents, held about two dozen Westerners a decade ago, when IS controlled a large swath of Syria and Iraq.

Several of the captives were killed in gruesome beheadings, including Americans James Foley and Steven Sotloff and Britons David Haines and Alan Henning.

Aine Davis, 38, was arrested at Luton Airport north of London on Wednesday night after arriving on a flight from Turkey and was charged with offences under the Terrorism Act, the Metropolitan Police Service said.

Davis is due to appear in a London court on Thursday. The Crown Prosecution Service said the charges relate to terrorism offences in 2014 and possession of a firearm for a purpose connected with terrorism.

Davis was arrested in Turkey in 2015 and convicted in 2017 of belonging to the Islamic State group. During his trial there, he denied being one of "The Beatles".

The group's four alleged members knew one another in West London before travelling to the Middle East and joining IS. Mohammed Emwazi, who carried out the executions and was nicknamed "Jihadi John", was later killed in a drone strike.

Two others, Alexanda Kotey and El Shafee Elsheikh, were captured by US-backed Kurdish forces in 2018 and are imprisoned in the U.S.

Australian Associated Press