The boss of MI5 has said he is “profoundly sorry” the Security Service did not prevent the Manchester Arena terror attack.

Director general Ken McCallum apologised after the public inquiry into the May 2017 atrocity revealed it could have been stopped if intelligence received months before had been acted on more swiftly.

Two pieces of information about suicide bomber Salman 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 the hearings into the bombing, which killed 22 people and injured hundreds – that he considered that did not present an “accurate picture”.

The officer admitted they considered a possible pressing national security concern on one of the pieces of intelligence but did not discuss it with colleagues straight away and did not write up a report on the same day.

In his 207-page report, Sir John said: “The delay in providing the report led to the missing of an opportunity to take a potentially important investigative action.

“Based on everything the Security Service knew or should have known, I am satisfied that such an investigative action would have been a proportionate and justified step to take. This should have happened.”

He said that if the intelligence had been followed up immediately it could have led to Abedi, 22, being followed to the parked Nissan Micra where he stored the explosive and later moved it to a rented city centre flat to assemble.

The chairman added that Abedi also could have been stopped at Manchester Airport on his return from Libya four days before the attack.

In response to Sir John’s findings, Mr McCallum said: “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.

“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.”