MI5 was “actively” investigating the Barking-based ringleader of the London Bridge atrocity at the time of the terror attack, according to an official assessment.

The report found that three terrorists involved in four attacks that hit Britain between March and June this year had at some point been under investigation.

The UK’s security policy was questioned after dozens of victims were killed or injured in separate incidents at Westminster, Manchester, London Bridge, and Finsbury Park.

MI5 and police launched independent reviews to examine what was known about the terrorists before they struck, decisions made on intelligence and possible areas for improvement.

An independent assessment of the findings by David Anderson QC, concluded that there is “no cause for despair”, saying most attacks continue to be successfully disrupted.

But he notes that, other than the case of Finsbury Park, it cannot be said that MI5 and police were “entirely blindsided”.

The report says: “Substantial and appropriate coverage was in place around key individuals and mechanisms designed to assess risk were working as intended.”

Khuram Butt, who led the gang behind the London Bridge van and knife attack in June, was the subject of an MI5 investigation from mid-2015 until the date of the deadly assault.

The report says material relating to Butt received in the two weeks prior to the attack added little to the intelligence picture and did not identify activity that led up to the attack.

It confirmed that he had worked at the Ummah Fitness Centre in Ilford and taught a Qu’ran class to young children.

The married father-of-two, whose youngest child was born less than a month before the terror attack, had attended school in Forest Gate and worked as an office manager for a branch of KFC for three years.

Another of the London Bridge gang, Youssef Zaghba, believed to be from Ilford, was placed on an EU warning list in March last year but a marker which would have automatically identified him as a national security risk was deleted by Italian authorities in January.

In June 2016, MI5 received an inquiry from Italian authorities about Zaghba but the agency has no record of responding - “noting by way of possible explanation that it arrived in the incorrect mailbox”.

The request was not chased up by Italian officials.

Zaghba, and the third London Bridge attacker Rachid Redouane, from Barking, were never investigated by MI5.