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

An inquiry found the bombing might have been prevented if MI5 had acted on key intelligence received in the months before the attack.

In the wake of the findings, director general Ken McCallum also expressed deep regret that such intelligence was not obtained.

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 May 2017 atrocity – that killed 22 people and injured hundreds – he considered that did not present an “accurate picture”.

In a statement, Mr McCallum said: “The terrorist attack at the Manchester Arena was a terrible tragedy. The bomber killed 22 innocent people and harmed many others. My thoughts are with the families and friends of those killed, and with all those whose lives were changed by this appalling act of terrorism.

“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.”

MI5 “continually” works to improve efforts to tackle terrorism and since the attack had made “more than 100 improvements”, he added.

“But we are determined to do more. As the chair now considers his recommendations, we will engage fully. 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.

“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,” Mr McCallum said.