As many as 23,000 people have appeared on the radar of counter-terror agencies, new figures laying bare the scale of the potential threat show.
In the wake of the Manchester suicide bombing it emerged that British authorities were grappling with 500 investigations into 3,000 individuals.
On Friday security sources confirmed a further 20,000 individuals were said to have been considered “subjects of interest” in the past, although the period the figures cover is unclear.
Anti-terror efforts came under fresh scrutiny following revelations that attacker Salman Abedi had been a “former subject of interest” to MI5 who was “subject to review”.
This would suggest the mass murderer was not regarded as posing an imminent threat when he detonated his device at the Manchester Arena, killing 22.
Security minister Ben Wallace told BBC Radio 4's Today Programme: “All those people are in the mix and they have to be looked at.
He added: “All of that is predominately underpinned by intelligence, which as I'm sure you will understand and the courts certainly understand, unfortunately the hardest part is we've got to convert intelligence into evidence if we actually want to deprive people of their liberty or take certain steps.”
A senior Whitehall source said on Thursday that 18 plots had been foiled since 2013 in Britain, including five since the Westminster atrocity in March this year.
The figures disclosed on Friday give an indication of what the source described as the “unprecedented” size of the current terror picture in Britain.
Round-the-clock monitoring of terror suspects is only possible in a handful of cases due to limited resources, the priority of which are determined on factors including credibility of intelligence and the apparent seriousness of the individual's intent.
PA
