Operation Temperer: What can the Army do to protect the UK from terror?

The Prime Minister appears to have launched a low level of Operation Temperer in which the armed forces help out, rather than take over the role of police firearms officers.

Raising the UK's terror threat level to critical, Mrs May said it would be up to police chiefs to decide where to deploy the military, though they are most likely to be used to guard top tourist attractions, airports and railway stations and sporting venues.

This weekend's FA Cup final at Wembley is a potential terrorist target.

:: LIVE: The latest updates on the Manchester attack from the Sky News team

Other troops are likely to be moved covertly around the country and be stationed, ready to respond to potential widespread attacks that might involve marauding gunmen.

In 1980, under a previous similar system, the police asked the SAS to take over a specific operation to storm the besieged Iranian embassy in London.

One reason they had to do so was that police firearms capability then was far less than it is now.

:: Bomb attack witness: 'It was absolute carnage'

Police today have far more trained firearms officers, thanks especially to extra Government funding in the wake of a recent increased threat from Islamic extremists.

That threat level has just been raised to critical, the highest, which means an attack may be imminent.

The Prime Minister said the Manchester attack may be part of a wider threat, suggesting clearly that police do not believe bomber Salman Abedi was acting alone.

:: Politicians unite to condemn attack

The threat level has been increased to the maximum twice before - in 2006, after the liquid bomb plot to blow up trans-Atlantic airliners, and a year later, after a failed attack to blow up gas cylinders outside a Central London bar and a similar attack on Glasgow airport.

Both times the level was reduced after a few days.

Mrs May has suggested that this time it could last several weeks.