BBC Home Affairs Correspondent
This is a major and very rare move by the UK's security chiefs. For the past few years, ministers, police chiefs and others have been at pains to try to warn the public that the threat faced by the UK was severe.
But they have steered clear of warning, even when a major plot was being tracked, that an attack could be close.
This time, they feel they have no choice other than to say it may be imminent. In short, nobody at this stage can say for sure whether Abedi acted alone or with the help of others.
They can't rule out whether there are other people out there.
You can read about the occasions when the threat level has been raised on the MI5 website.
In the meantime, what does it mean for us, the public? Some of us will see the Army in key locations that need guarding, so that armed police can be freed to focus on policing rather than guarding.
We should expect additional time-consuming security checks at ports and so on. The aim for security chiefs is to ratchet up the security while keeping the country moving.