Britain reduces terror threat level from 'critical' to 'severe' But British PM Theresa May urged residents to remain vigilant.

 -- British Prime Minister Theresa May said Saturday morning that the country's terrorism threat level has been reduced from its top level of "critical" to "severe."

The change indicates an attack is highly likely, not imminently expected.

The level was raised to critical after Monday's bomb attack at an Ariana Grande concert in Manchester, which left 22 people dead.

May cited progress in the investigation for the change in threat level, but urged people to remain vigilant.

Following May's announcement, London mayor Sadiq Khan tweeted, "Security plans will remain in place this weekend -- including additional policing for major events and the army helping with police guarding duties."

Khan continued, "I encourage all Londoners to remain vigilant and report anything suspicious to the police."

A previous version of this story stated the threat level had been reduced to "imminent" instead of "severe."