A 26-year-old man has been arrested at Heathrow Airport after he arrived back in the UK from Turkey, hours after authorities in Ankara said a British Islamic State suspect had been deported.
The man was arrested by counter-terrorism police on suspicion of Syria-related terror offences.
The Metropolitan Police said the man is being held "on suspicion of preparation of terrorist acts under section 5 of the Terrorism Act 2006".
It added that the arrest was "Syria-related".
Earlier, Turkey's Interior Ministry said it had deported eight so-called Islamic State suspects, including the Briton and seven Germans.
It did not identify any of the suspects and the Home Office in London also refused to disclose any details, saying it did not comment on individual cases.
Speaking after the man's arrest, Home Secretary Priti Patel said any former members of so-called Islamic State will be "arrested and investigated by counter-terrorism police" if they return to the UK.
Turkey has declared its determination to deport alleged foreign IS members who are held in its prisons or in parts of Syria it controls.
That came after Ankara was condemned by some western governments for sending forces to parts of north east Syria to drive out Kurdish fighters it considers to be terrorists even though they were allies in the US-led push to defeat IS in the region.
Turkey has already deported a US national and IS suspect who was stuck in a no man's land between Turkey and Greece after Athens refused him entry. Ankara said the United States had agreed to take him back.