Greater Manchester is one of three areas piloting a new anti-terror scheme in which MI5 will share information about "subjects of interest" with local police and councils.
It follows a review of recent attacks including the Manchester Arena bombing.
Those behind the scheme say greater intelligence sharing will minimise the threat posed by suspects who are rapidly radicalised before security services can detect the change.
Theresa May said the new strategy "has a much greater focus on systemic co-ordination across the public sector".
Salman Abedi, the Manchester bomber, was categorised as a "closed subject of interest" at the time of the attack, and so not under active investigation.
There are an estimated 20,000 individuals who have previously featured in terrorism probes, either as active targets or on the periphery of inquiries.
Officials emphasised that the new project will be on a much smaller scale - with the number of cases where intelligence could be shared expected to be in the low hundreds over the next year.