Police yesterday carried out a controlled explosion at the doorstep of his home during raids around the city.

Britain's Joint Terrorism Analysis Centre last night raised the terror threat level to 'critical', its highest level.

The threat level has only been raised to 'critical' twice since the system was introduced on August 1, 2006.

Critical is the highest threat level under the system which is 'designed to give a broad indication of the likelihood of a terrorist attack'.

Low means an attack is unlikely while moderate means an attack is possible, but not likely.

Security services are trying to establish whether Salman worked alone or was part of a wider network that helped him with the bomb.

A school friend told The Times that Abedi had returned to Libya in the past week.

She said: 'That's why the terrorists will never win, and we will prevail.' Tonight's announcement comes after ISIS claimed responsibility for the worst terror attack Britain has seen since the 7/7 London bombings.

The suicide bomber, Salmon Abedi, was known to the security services but was not part of any active investigation or regarded as a high risk.

Just one week after its launch, the levels were raised to critical when police uncovered a plot to smuggle explosives on passenger jets travelling between the UK and US.