A number of shortcomings in the handling of Salman Abedi before he launched a suicide attack at an Ariana Grande in May last year, killing 22 people, were detailed by Parliament's Intelligence and Security Committee (ISC).
Abedi, 22, first came to the attention of MI5 in December 2010 and was briefly investigated by the agency in 2014.
The ISC assessment said:
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- Abedi visited an extremist contact in prison on more than one occasion but no follow-up action was taken by either MI5 or police;
- MI5 decided not to place travel monitoring or restrictions on Abedi, meaning he was allowed to return undetected to the UK in the days before he carried out the attack;
- MI5 systems moved too slowly after Abedi's case had been flagged for review;
- Abedi was not at any point considered for a referral to the Prevent anti-terror scheme.
ISC 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 in relation to the attacks last year had previously been raised by the committee in its reports on the 7/7 attacks and the killing of Lee Rigby.
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 tragic events must now result in real action."
The ISC, which has access to top-level security officials and classified material, reviewed the five attacks that hit Britain last year.
In total 36 innocent people were killed.
In addition to the Manchester fatalities, five victims died in the Westminster attack 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.
Five people from the North East were among those who died in the Manchester attacks.
They were; Chloe Rutherford, 17, and boyfriend Liam Curry, 19, from South Shields; Hartlepool born Jane Tweddle-Taylor, 51; and Philip Tron, 32, and his partner’s daughter Courtney Boyle, 19, from Gateshead.
The events last year prompted intense scrutiny of Britain's counter-terrorism apparatus after it emerged that in a number of cases, the perpetrators had previously appeared on the radar of agencies.
Prior to his attack, Abedi had travelled to Libya.
The ISC questioned the decision not to use travel monitoring and travel restriction capabilities in the case, adding: "We recognise that 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."
The committee also said there appeared to have been "fundamental failings" in the way police and the Home Office handled Parsons Green attacker Ahmed Hassan.
But Mr Grieve said the case was not fully examined in the assessment because the Home Office failed to provide full evidence in time despite multiple requests.
The report found that the system for regulating and reporting purchases of ingredients used to make explosives was "hopelessly out of date", and called on the business community to exert pressure on communications firms to stop their sites being used as a "safe haven" for terrorists.
Security chiefs say they are operating at an unprecedented pace to head off the threat.
Since March last year, 17 plots have been foiled, while police and MI5 are mounting a record 700-plus live investigations.
There are around 3,000 active "subjects of interest", plus more than 20,000 closed SOIs who have at some point featured in terrorism probes.
Home Secretary Sajid Javid said: "Following the attacks, the Government, police and MI5 undertook a series of rigorous reviews to ensure we are all doing everything we can to tackle the evolving threat of terrorism.
"As a result, 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."
National head of counter-terror policing Assistant Commissioner Neil Basu said: "We would like to reassure the public that ever since the attacks of last year we have sought to learn from what happened before, during and afterwards, and improve our wider operating model and ways of managing and mitigating the risk from terrorism.
"Working evermore closely with the security service and learning our lessons, we will do everything we can to reduce the chances of this happening again."