An 18-year-old man has been arrested on suspicion of terror offences in connection with Friday’s attack on a London tube.
The man was detained in the port area of Dover this morning by Kent police and is being held at a local station.
Metropolitan police said the arrest was “significant”, but the terror threat level remains at “critical”.
The London train has been reopened after the blast that left 30 people injured.
Prime Minister Theresa May has said the threat was now critical, meaning an attack is expected imminently.
Police said some 1,000 armed officers would be seen across the country after military assistance was requested.
May said the military would be providing support to police and would replace officers on guard duty at national infrastructure sites that are not accessible to the public.
Police Scotland said it would be increasing the number of armed officers on patrol, particularly at key locations and crowded places.
so-called Islamic State has said it was behind the attack.