The Home Office said it "remains committed" to supporting the search for Madeleine McCann, who disappeared from her family's holiday apartment in Portugal's Praia da Luz over six years ago.
A Home Office spokesman said: "We have always said we would provide the Metropolitan Police with the resources they need to investigate her disappearance".
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The threat level has been raised to critical by MI5's Counter-Terrorism Analysis Centre.
Prime Minister Theresa May announced the decision from Downing Street.
A man, apparently armed with a small axe and a large stick, was detained close to the event and police cleared the area as a precaution.