Prime Minister Theresa May has tonight announced that Britain is on the highest terror alert of “critical” – meaning another attack is expected imminently.

The military are set to bolster security as police continue their hunt for the Parsons Green bomb “suspects” tonight.

A manhunt is under way after an improvised explosive device sent a fireball through a packed London Underground train carriage in south west London during this morning’;s rush hour.

Assistant Commissioner Mark Rowley of the Metropolitan Police suggested there may have been more than one person involved.

He said police were “chasing down suspects” while giving a statement outside Scotland Yard.

Assistant Commissioner Rowley, who is the UK’s most senior anti-terror police officer, said: “Somebody has planted an improvised explosive device on the Tube – we have to be open-minded at this stage about him and about potential associates.”

The Islamic State (IS) terror group has claimed responsibility for the attack through its Amaq news agency, according to the US-based Site Intelligence.