Abedi, 22, first came to the attention of the security services December 2010 and was briefly investigated by in 2014, three years before he detonated a bomb at Manchester Arena, killing 22 people.
An assessment by Parliament’s Intelligence and Security Committee found that Abedi visited an extremist contact in prison more than once but that neither police nor MI5 took any follow-up action.
They concluded that MI5 systems moved too slowly after his case had been flagged for review, and that he was at no point considered for a referral to the Prevent anti-terror scheme.
The committee’s chairman, Dominic Grieve, said: “What we can say is that there were a number of failures in the handling of Salman Abedi’s case and, while it is impossible to say whether these would have prevented the devastating attack, we have concluded that, as a result of the failings, potential opportunities to prevent it were missed.”
Mr Grieve said it was “striking” how many of the issues which arose last year had been raised previously by the committee in its reports on the 7/7 attacks – in which three suicide bombers from West Yorkshire and a fourth from Luton killed 52 people in central London – and the killing of Fusilier Lee Rigby, near Woolwich Barracks.
The Conservative MP said: “We have previously made recommendations in all of these areas, yet the Government failed to act on them.”
He noted that both MI5 and counter-terror police have been “thorough in their desire to learn from past mistakes”, adding: “The lessons from last year’s events must now result in real action.”
Mr Grieve’s committee, which has access to top-level security officials and classified material, reviewed the five attacks in Britain last year. Five victims died at Westminster in March, eight at London Bridge in June, and one at Finsbury Park in the same month. Then in September, a bomb partially exploded on a tube train at Parsons Green, injuring 51 people.
It emerged previously that in several cases, the perpetrators had already appeared on the radar of agencies.
The committee questioned the decision not to use travel monitoring in the case of Abedi, who flew to Libya and returned undetected to the UK in the days before he carried out the attack.
It said: “There still may not have been sufficient time to identify or act on his attack planning. It would, nevertheless, have provided more of an opportunity.”
Mr Grieve said: “MI5 have since admitted that given the information they had on Abedi, they should have done so, and they have now revisited their policies in this respect.”
The committee also said there appeared to have been “fundamental failings” in the way police and the Home Office handled the Parsons Green attacker, Ahmed Hassan. But Mr Grieve said the case was not fully examined because the Home Office had failed to provide full in time.
The Home Secretary, Sajid Javid, said: “We have updated our counter-terrorism strategy, introduced new legislation to allow threats to be disrupted earlier and have increased information sharing with local authorities.
“We are also ensuring technology companies play their part by stopping terrorists from exploiting their platforms.”