British Prime Minister Theresa May says the country's threat level from terrorism had been reduced from its top level of "critical," which indicates an attack is imminent.

May says it now stands at "severe," meaning an attack is highly likely.

The level was raised to critical after Monday's bomb attack in Manchester, which killed 22 people.

May says major progress has been made in the investigation but that people should remain vigilant.

British police are preparing major security operations as people across Britain begin celebrating a three-day holiday weekend.

Five days after a bomber killed 22 people at a concert in Manchester, more than a thousand armed police are on standby as major events including the Football Association Cup Final and the Premiership Rugby Final are expected to draw tens of thousands of people.

Despite the country being placed on alert, police have urged people to go out and enjoy themselves.