- Video report by ITV News Security Editor Rohit Kachroo
A report reviewing counter-terrorism intelligence said that it is "conceivable" that the Manchester attack might have been avoided.
The report, commissioned by Home Secretary Amber Rudd, assessed the use of intelligence by MI5 and Counter Terrorism Police in the run up to the four attacks in London and Manchester this year.
The 'Attacks in London and Manchester, March-June 2017' report assessed internal reviews carried out after the attacks in Westminster, London Bridge, Manchester Arena and Finsbury Park.
It said: "It is conceivable that the Manchester attack in particular might have been avoided had the cards fallen differently.
"While investigative actions in all cases were for the most part sound, learning points have emerged which should help avert future terrorist attacks."
The report found that Manchester bomber Salman Abedi was a former "subject of interest" (SOI) to intelligence officers on the basis of supposed contact with a Daesh figure in Libya.
However his status as an SOI was closed on the same day when it transpired that any contact was not direct.
It said: "On two separate occasions in the months prior to the attack, intelligence was received by MI5 whose significance was not fully appreciated at the time.
"It was assessed at the time to relate not to terrorism but to possible non-nefarious activity or to criminality on the part of Salman Abedi. In retrospect, the intelligence can be seen to have been highly relevant to the planned attack."
Abedi was also one of a "small number of individuals" out of 20,000 closed SOIs who were identified as meriting further examination.
A meeting arranged before the attack was scheduled to take place on May 31 where Abedi's case would have been considered but the "attack intervened" on May 22, the report said.
London Bridge attacker Khuram Butt was an active SOI and Khalid Masood who carried out the Westminster attack was a closed SOI.
Outlining the review findings, Home Secretary Amber Rudd told MPs that blame for the "cowardly" attacks "lies squarely" with the terrorists and those who encouraged them and paid tribute to the work that police and MI5 carried out.
She said that they have now disrupted 22 Islamist terrorist attacks since murder of Lee Rigby in May 2013.
A total of nine terrorist attacks have also been thwarted since the Westminster attack in March this year, she said.
That figure had earlier been confirmed by MI5 director general Andrew Parker.
Outlining the review findings, Ms Rudd told the Commons as Abedi was a closed SOI, he was not under active investigation.
She added: "In early 2017, MI5 nonetheless received intelligence on him, which was assessed as not being related to terrorism.
"In retrospect, the intelligence can be seen to be highly relevant.
"Had an investigation been reopened at the time, it cannot be known whether Abedi's plans could have been stopped.
"MI5 assess that it would have been unlikely."
But she said the report shows that intelligence officials "got a great deal right".
The Home Secretary added that London Bridge attacker Khuram Butt was an "active" SOI who had been under investigation since mid-2015.
Ms Rudd said: "A number of different investigative means were deployed against him, but they did not reveal his plans.
"His two conspirators had never been investigated by MI5 or CT policing.
"In regards to Finsbury Park, neither MI5 nor the police had any intelligence about this attack.
"Taken as a whole, MI5 and CT policing conclude they could not, quote, 'find any key moments where different decisions would have made it likely that they could have stopped any of the attacks'."
Ms Rudd also outlined the terror threat facing the UK, saying there was "simply more terrorist activity" which had been partly inspired by more terrorist propaganda and instruction videos online.
David Anderson QC, who wrote the report, said the review was to identify "systemic improvements rather than apportion blame".
"Despite elevated threat levels, the fundamentals are sound and the great majority of attacks continue to be thwarted. But the shock of these incidents has prompted intensive reflection and a commitment to significant change," he said.
Following their own internal reviews, MI5 and police have identified 126 recommendations to strengthen their intelligence capabilities.
These include the need to use data more effectively, share knowledge more widely, improve their own collaboration and to assess and investigate terrorist threats on a uniform basis, whatever the ideology that inspires them.
Responding to the report, the head of Scotland Yard Cressida Dick called for new technology, better infrastructure and resources in the fight against terror.
She said the terror threat is posing "significant challenges" for police and security services.
In its full response, the Metropolitan Police said: "Although we work tirelessly to keep the country safe the reality of the terrorist threat means that we will not stop every attack."
Andrew Parker, the Director General of MI5, said the agency is committed to implementing the recommendations identified in the report.
He added: "We and our partner agencies used the harsh light of hindsight under independent challenge to ensure we squeezed every drop of learning out of these dreadful events.
"Our unrelenting focus remains on doing everything in our power every day to keep the country safe."
- Security Minister Ben Wallace reacts to the report