The security services will be forced to answer more questions about Manchester Arena suicide bomber Salman Abedi following pressure from families of the victims.
MI5 has previously admitted Abedi had come across its radar 18 times since 2010.
The Libyan-heritage Fallowfield-raised Salford University student was made a 'subject of interest' in 2014 - but the file was dropped four months later.
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The security services also uncovered links to six other people of interest to them and, even a few months before the Arena atrocity, more information came to light which had flagged him up for potential further investigation, although the meeting to discuss his case was scheduled to take place nine days after the bombing.
MI5 has also admitted a 'missed opportunity' to stop Abedi when he landed at Manchester Airport just a few days before he launched his attack
Following two weeks of evidence in November held in secret to protect national security, last month the chairman of the inquiry Sir John Saunders published a 15-page 'gist' of the secret session he considered safe to be released to the public.
The document revealed more of the information counter terror police and MI5 had on Abedi, and the pressure that the security services were under, prior to the devastating May 2017 attack, which claimed 22 lives and left a thousand others injured.
But it did not expand on two key pieces of intelligence on Abedi MI5 had received prior to the attack, 'the significance of which was not fully appreciated at the time'.
MI5's current policy would have led to Abedi being investigated as a 'low-level lead' following the first of these pieces of intelligence, according to the gist.
And two MI5 witnesses have also testified that further investigative steps may have been taken had the tip-offs come with 'further context' amid concern about the sharing of information between the security services.
The published gist document prompted a series of written follow-up questions from the families and a formal request that two witnesses, a senior MI5 official known only as Witness J and Detective Chief Superintendent Dominic Scally, the head of Counter Terrorism Policing North West, should be recalled to answer them.
But inquiry chairman Sir John has now ruled the two witness should not return as they would be prevented from answering some of the questions.
"That would be deeply upsetting for the bereaved families, unsatisfactory for the witnesses and would frustrate the process, as it would be of no assistance to me," stated Sir John in his ruling published on the inquiry website.
He went on: "I well understand the desire of the bereaved families to have those questions asked and answered, but everyone must appreciate that if answers could have been given to those questions without damaging national security, they most likely would already have been provided."
Sir John stated the scope for 'permissible answers' was 'narrow' and that follow-up questions were 'likely not to be capable of immediate, if any, answer'.
However, Sir John ruled the witnesses could provide written answers to some of their questions. He said he had been able to reduce and simplify the questions, ruling out entirely questions which 'cannot be answered for national security considerations'.
Among the permitted questions are two enquiries about Didsbury Mosque: namely, when did counter terrorism policing try to engage the mosque to help prevent extremists travelling to Syria and why was DCS Scally of the view that the mosque's response was 'less positive' than two others that had been approached, Al-Furqan and El-Noor mosques?
The families have also been permitted to ask who in MI5 treated Abedi as a 'de facto' or 'tier two' subject of interest - the gist reveals that three MI5 witnesses had given evidence that they did not recognise these concepts.
They also asked whether the MI5 official who had adopted this approach was aware of the wealth of Islamic State material found on Abedi's older brother Ismail when he was stopped by counter terror police at Heathrow Airport in September 2015 on his way back from a honeymoon in Indonesia.
The families have also asked about the number of other 'subjects of interest' with whom Salman Abedi had contact.
They have asked when MI5 and counter terror police bosses realised there were 'shortcomings' in communication between the two bodies and when was action taken to address them.
The families have also asked about a joint policy agreement between MI5 and counter terror cops around the sharing of intelligence which did not exist before the attack but does now. They have asked whether the new agreement has improved the working relationship between them.
The gist reveals that Libya was among the top four priorities for counter terror policing in 2017 and 2018 alongside Syria, online radicalisation and vulnerability arising from mental health. The families are asking if this was also the position of MI5.
Another question the families have been allowed to put is whether the earlier 'attribution' of a mobile phone linked to Salman Abedi, in 2014, would have made any difference to assessments the security services made about him.
Sir John has ruled the answers to these questions should be submitted by March 11 and will be published, subject to any formal objection from the security services.
The families and other 'core participants' will then be invited to make closing submissions on the role of the security services when the inquiry resumes on March 14.
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