They touched down at Luton Airport on Wednesday night and were later taken into custody, with detectives continuing to question them at a police station in Bedfordshire.
Scotland Yard said the suspects, all Sri Lankan nationals, had been detained under the terrorism act.
Police at the airport were made aware of the men the next day, Thursday 11 April and subsequently stopped them under schedule seven of the Terrorism Act 2000, confirmed a police spokesman.
None of the men have been identified as they have not been charged. Inquiries are ongoing.