It is the first time Britain has been on maximum terrorist alert since 2007, when a blazing car loaded with gas canisters was driven into Glasgow Airport.

A 23-year-old man was arrested in Manchester in connection with the attack and was being questioned.

Intelligence experts believe the device detonated at a concert by the pop singer Ariana Grande at the Manchester Arena on Monday night was so sophisticated that Abedi must have either been given specialist training abroad or used a bomb made by a technician who has not yet been captured.

It emerged that Abedi had travelled to Libya, raising fears he had been trained there and posing questions for the security services on whether he should have been tracked. Police confirmed that an eight-year-old girl was among those killed.