Details of the alleged terror plot were set out to Cabinet members on Tuesday during a briefing by Andrew Parker, the head of MI5. Mr Parker revealed that British intelligence had foiled nine terror plots in the past 12 months.

The disclosures about the charges came just hours after an official report into the Manchester terror attack revealed that the suicide bomber had been flagged for closer scrutiny by security services and that the atrocity could have been averted “had the cards fallen differently”.

Internal reviews into the police and MI5’s handling of the four terrorist 
attacks in Britain this year also revealed one of the London Bridge attackers had been under active investigation by the Security Service.

The Westminster Bridge attacker, Khalid Masood, had also watched suicide attack videos on YouTube in the days before he carried out his assault.

David Anderson QC, a former terrorism law reviewer asked by the Home Secretary to independently check the secret internal reviews, said they were “no cause for despair” and that most attack plots continued to be broken up.

In response to his 61-page report, Amber Rudd, the Home Secretary, said the blame for the attacks “lies squarely” with the terrorists.

The reviews found that 22-year-old Abedi had previously been a MI5 suspect, but was not under active investigation when he blew himself up among the crowd at an Ariana Grande concert.

In advance of the attack, officers had on two separate occasions received unspecified intelligence on him “whose significance was not fully appreciated at the time” and which could have led to his case being reopened.

“In retrospect, the intelligence can be seen to have been highly relevant to the planned attack,” the report said.

Mr Anderson concluded that while it was “unknowable” if reopening the investigation would have thwarted Abedi, it was “conceivable that the Manchester attack in particular might have been averted had the cards fallen differently”.

Between March and June, London and Manchester experienced four attacks killing a total of 36 people and wounding another 200.

Abedi had first become an MI5 “subject of interest” in 2014, but it transpired he had been mistaken for someone else and his case was closed. It was reopened the following year
 on mistaken intelligence that he had contacted an Islamic State figure in Libya.

But though his case remained closed from that point, Abedi “continued to be referenced from time to time in intelligence gathered for other purposes. In two separate instances before the attack, intelligence was received that was “assessed at the time to relate not to terrorism, but to possible non-nefarious activity or to criminality”.