The terror-related arrest of a teenager at Heathrow airport was not made in response to any immediate risk or threat, police said.
A 19-year old man has been arrested on suspicion of "preparing for acts of terrorism" in relation to Syria.
The man is currently held at a police station in London.
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The chancellor now faces the prospect of forming a minority government or calling for a snap election.
The 73-year-old said a return is unlikely "given the politicised atmosphere that seems to have afflicted our set."
Chancellor Philip Hammond said the UK will make a financial proposal to the EU in the next three-and-a-half weeks.