Speaking ahead of the release on Thursday of the Manchester Arena Inquiry's third and final report, which will look into role the security service played in the months and years leading up to the attack, Andrew Roussos wants MI5 bosses to "admit" it made mistakes, and let Abedi "slip through the cracks".

He told Sky News: "Salman Abedi should have been stopped before he got to that arena and that's MI5's job.

"They know the threat, they know what these people do and don't do, they know where to look and not to look, and they were more prepared than what Manchester was that night - so MI5, for me, have got blood on their hands."

He added: "I want them to admit that with all the signs and what they knew about this family and Salman Abedi that they let it slip through the cracks, because they did."

Over months of evidence behind closed doors, the inquiry heard from the secret service about its timeline of events in identifying Salman Abedi as a potential threat and why it didn't escalate his case.

The third report is also due to focus on the radicalisation of the Abedi family and Salman's links with convicted terrorists.

It was revealed during the evidence that MI5 received highly relevant intelligence about Salman Abedi, twice, in the months before his attack, but it was treated as "non-terrorist criminality".

With hindsight, the security service says the significance of that information at the time wasn't fully appreciated.

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MI5 'obsessed with secrecy and an unhealthy organisation'

A lawyer for the families of five victims believes MI5 is "too slow and inadequate".

Nicola Brook told Sky News: "This was an individual they had information on since 2010 - it isn't somebody who just dropped onto their radar. They knew about his familial links with extremism, they knew the risk that was posed within that community, they knew about his links with convicted terrorists - but none of that was considered sufficient enough to conduct further investigation."

She added: "MI5 is obsessed with secrecy, and it's an unhealthy organisation that shies away from any kind of scrutiny. And the question we've been asking is: is this about national security or national embarrassment?"

In response to these allegations, the justice secretary, Dominic Raab, accepted failures in the past, but fully supported the secret service.

He said: "I'll allow the government to respond to the report in the usual way but my experience of working with all of the intelligence agencies in this country is we're incredibly well served.

"Of course there are always lessons to be learned, and they are very open to that and, actually, they service this country by definition, unseen."

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Inquiry chair 'won't pull his punches'

Sir Mark Lyall Grant was the UK's national security adviser in the two years prior to the attack and told Sky News that MI5 will take on any recommendations suggested by the inquiry chairman.

He said: "MI5 and other intelligence agencies receive hundreds if not thousands of pieces of information every day, and sifting that and analysing that, mistakes can be made and things can slip through the cracks.

"Sir John Saunders, who was very critical of the response of some of the emergency services after the attack, will be very trenchant and straightforward in his findings that he produces about the intelligence services in advance of the attack.

"So he won't pull his punches and MI5 have said publicly they don't want him to pull his punches."

While the third and final report from the public inquiry into the Manchester Arena terrorist bombing due out on Thursday will crucially address what was known by the security service and counter-terror policing prior to the atrocity, wide redactions are expected with certain evidence heard behind closed doors for reasons of national security.

Deputy Prime Minister Dominic Raab told Sky News: "My experience of working with all the intelligence agencies in this country is we're incredibly well served.

"Of course there are always lessons to learn and they are very open to that, but actually they serve this country by definition unseen and we're very lucky to have such professional intelligence agencies in this country."

A government spokesperson said: "Our thoughts remain with those who were killed or had their lives changed forever at the Manchester Arena attack.

"The participants to the inquiry will respond publicly after the publication of the chair's report."