A woman has been arrested at Luton airport on suspicion of Syria-related terror offences.
West Midlands Police counter-terrorism detectives detained the woman, 25, at around 11:30am as she left a flight which had returned to the UK from Turkey's capital, Istanbul.
Her arrest on suspicion of preparing for acts of terrorism is part of an ongoing intelligence-led operation, according to police.
The woman from Haringey, north London, is being held at a police station in the West Midlands.
The incident comes after the threat level in the UK was raised from substantial to severe earlier this year, amid concerns over the hundreds of Britons travelling to Iraq and Syria to join the conflict in the region.
In late November, a separate man and woman were also arrested at Heathrow Airport on suspicion of terror offences related to Syria.
The rise of Isis, which has taken hold of swathes of Iraq and Syria over the past year, has attracted thousands of foreign jihadists to its cause, including more than 500 Britons, with about 250 believed to have returned home.
A new Counter-terrorism and Security Bill, containing a range of draconian powers including new orders that can block suspected British fighters from returning to the UK, was introduced to Parliament this year.
The bill introduced by Home Secretary Theresa May is intended to stop Britons travelling Syria and Iraq to fight for or against Isis, and to prevent veterans of those conflicts from returning unless they agree to comply with the authorities under Temporary Exclusion Orders.
But some have criticised the bill, with David Anderson, Britain's independent reviewer of terrorism laws, telling parliament's Committee on Human Rights in November that he was concerned about the judicial checks for the latter proposal.
Additional reporting PA