An inquiry found the bombing might have been prevented if MI5 had acted on intelligence received in the months before the attack when two pieces of information about Abedi were assessed at the time by the security service to not relate to terrorism.

But inquiry chairman Sir John Saunders said, having heard from MI5 witnesses at secret hearings, he considered that did not present an “accurate picture”.

In the wake of the findings, Mr McCallum made a rare public statement in which he apologised for the failings identified and insisted improvements had been made.

Speaking in Manchester on Thursday, he said: “Having examined all the evidence, the chair of the inquiry has found that ‘there was a realistic possibility that actionable intelligence could have been obtained which might have led to actions preventing the attack.’

“I deeply regret that such intelligence was not obtained.

“Gathering covert intelligence is difficult – but had we managed to seize the slim chance we had, those impacted might not have experienced such appalling loss and trauma. I am profoundly sorry that MI5 did not prevent the attack.”

Neither MI5 nor the Home Office would say whether anyone at the security service had faced disciplinary action.

Questions also remain about Abedi’s links with the late Muslim preacher Mansour al-Anezi, who he visited before his death.

The inquiry’s report lists him as an associate of Abedi who was arrested in 2008 as part of the investigation into failed Exeter suicide bomb attacker Nicky Reilly.

Abedi and his brother Hashem – who was jailed for life after he was convicted of assisting Manchester Arena plot – were “in contact” with al-Anezi between October 2016 and January 2017 and the pair went to his funeral, according to the findings.

While the details of the relationship were “not known”, the inquiry heard police considered it a “connection of significance”.

Earlier this week, the BBC reported al-Anezi had been suspected by MI5 of radicalising people more than a decade earlier. It is not known whether the inquiry considered the unconfirmed reports.

Abedi was recorded as a subject of interest (SOI) in 2014 and investigated by MI5 but later deemed “low risk”. He was identified on six occasions as a contact of suspected extremists between 2013 and 2017.

The security service investigates around 3,000 active SOIs when it believes someone may pose a threat and has about 600 live investigations at any one time. Significant numbers of SOIs are overseas.

They are ranked by priority and resources will be dedicated to them depending on the level of threat they are judged to pose.

When they are no longer deemed to pose a threat, they are downgraded and marked as a “closed” SOI, joining a list of around 40,000 people who have at some point featured in probes.

Mr McCallum did not take questions on the findings but in his recorded statement said since the attack MI5 has made “more than 100 improvements” as it “continually” works to better efforts to fight terrorism.

The Counter Terrorism Operations centre (CTOC), which opened in June 2021, is considered by officials to be a major step forward in that mission because it brings together police, intelligence agencies, government departments and other elements of the criminal justice system involved in counter-terror operations in one place for the first time.

The move was in a bid to speed up the response to threats and improve how terrorists are monitored.

MI5 will “engage fully” with the inquiry’s recommendations and stressed that the security service was “determined to do more”, Mr McCallum said, adding: “Where there are opportunities to strengthen the UK’s defences further, MI5 will act.

“We will continue to do everything in our power to keep our country safe from hidden threats.

Describing the attack as a “terrible tragedy”, Mr McCallum said his thoughts were with the “families and friends of those killed, and with all those whose lives were changed by this appalling act of terrorism.”

“MI5 exists to stop atrocities. To all those whose lives were forever changed on that awful night: I am so sorry that MI5 did not prevent the attack at the Manchester Arena,” he added.

Head of Counter Terrorism Policing Matt Jukes also promised to “act quickly” on the inquiry’s findings and insisted officers remain “relentless in our work to keep the public safe.”

He said: “I am sorry that, despite our determined partnership (with MI5), we did not stop the loss of life, nor the injury and trauma that happened close to here, almost six years ago.

“I want those who have lost loved ones to know that their loss has steeled us to ensure our counter terrorism partnerships are stronger, faster and more effective.”

MI5 and the police disrupted at least 37 late-stage terror attack plots since the start of 2017.

Home Secretary Suella Braverman said: “I am committed to working with MI5, policing and partners to study the recommendations. Together we will do everything possible to prevent a repeat of this horrifying attack.”

Security minister Tom Tugendhat said he shared the “sorrow” expressed by MI5, adding: “I know that all of us are working to make sure that the improvements that need to be made are made.”