A 32-year-old man suspected of fighting in Syria was arrested at Luton Airport December 27 upon his return to the UK.
A Metropolitan Police spokesperson said the man was stopped on Thursday after arriving on a UK-bound flight.
"He was arrested on suspicion of preparation of terrorist acts and taken to a police station in the Bedfordshire area, where he remains in custody. The investigation is being conducted by detectives from the Met's Counter Terrorism Command. The arrest is not related to any offences at the airport. Enquiries continue," a Metropolitan Police spokesperson said.
Officials wouldn't confirm which group the suspect is accused of joining in Syria, although anyone travelling to the country to fight can be arrested irrespective of their cause.
Of an estimated 900 individuals who've travelled from the UK to Syria since 2011 who pose a "national security concern", around 20 percent have been killed in fighting and 40 percent have returned to their home country.
In October, Minister of State for Security Ben Wallace warned returning Daesh [banned in Russia] fighters could represent a significant terror threat, as they may bring back knowledge learned on battlefields in Syria with them.