By Andy Hayes, news reporter
A man has been arrested at Heathrow airport after Turkey said it had deported a British Islamic State suspect.
Scotland Yard said the 26-year-old was held "on suspicion of preparation of terrorist acts under section 5 of the Terrorism Act 2006", adding that it was "Syria-related".
The man arrived on Thursday "on an inbound flight to the UK from Turkey", it added.
The Home Office has refused to disclose any details, saying it does not comment on individual cases.
Ankara also said on Thursday it would be sending an American fighter back to the US after he was refused entry to Greece, leaving him in a heavily militarised border zone for three days.
The US has agreed to take the militant and to give him travel documents, Turkey's interior ministry said.
According to Turkish media, he is 39, of Jordanian background, and called Mohammad Darwis B.
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The decision came a day after Turkey's President Erdogan held talks with Donald Trump in Washington.
Ankara has also deported seven German nationals.
Turkey's state-run Anadolu news agency said two men, four women and an infant were taken onto the tarmac at Istanbul airport in a vehicle belonging to Turkey's migration agency, and boarded a plane for Berlin.