MI5 has thwarted five terror plots in the last two months alone as the country faces an “unparalled” threat from Islamist extremists, according to a senior Whitehall source.
Officials say a total of 18 plots have been foiled since 2013.
The scale of the menace was disclosed by counter terrorism officials after it recently emerged that the Manchester suicide bomber, Salman Abedi, was known to the security services before he carried out the horrific attack.
The danger posed by Abedi was reportedly flagged up to authorities on at least five separate occasions before he blew himself up at a pop concert at Manchester Arena killing 22 people.
The source said: “Abedi was one of a larger pool of former subjects of interest whose risk remained subject to review by MI5 and its partners."
The source added that if a former subject seems to show a risk of heading back into terrorism “MI5 can consider re-opening the investigation, but this process inevitably relies on difficult professional judgements based on partial information.”
A former senior security figure added: "For every suspect that appears to be high priority another has to be pushed down the list.
"So who not to investigate urgently is as important a decision as who might be worth investigating."
Prime Minister Theresa May raised the terror threat in the UK to “critical” on Tuesday, meaning another attack could be imminent.
