Some evidence at the Manchester Arena inquests is to be heard in private after a coroner ruled making it public would "assist terrorists" in carrying out similar atrocities.
Sir John Saunders has granted applications by the Home Office and police for public interest immunity (PII) on the grounds of protecting national security.
Part of last week's pre-inquest review hearing into the deaths of 22 people at the end of an Ariana Grande concert on May 22 2017 was held in private to consider the PII applications over material related to the issue of whether the attack by suicide bomber Salman Abedi could have been prevented by the authorities.
On Friday, retired High Court judge Sir John published an open ruling in which he set out the reasons for his decision.
He said: "I have upheld the claims for PII by both the Secretary of State and CTP (Counter Terrorism Police North West).
"I have done that because I am satisfied, having heard the justifications for them, that to make public those matters would assist terrorists in carrying out the sort of atrocities committed in Manchester and would make it less likely that the Security Service and CT police would be able to prevent them.
"The balancing exercise strongly favours the material in question for not being disclosed. I will, of course, keep this ruling under review."
He added that, as a consequence, his provisional view was that an "adequate investigation" could not be conducted within the frameworks of the inquests.
It is thought that discussions of whether the inquests will be converted into a public inquiry will be raised at the next scheduled pre-inquests hearing on October 7.
A total of 22 people were murdered by Abedi, 22, at the end of an Ariana Grande concert at Manchester Arena on May 22 2017.
Last week Sir John reassured families attending the hearing at Manchester Town Hall that public interest immunity "will not be used as a device for covering up responsibility" and that he will do his "very best" to ensure that does not happen.
Lawyers for the families of the 22, who have not seen the evidence the authorities want to withhold, stressed the need for as open an inquest process as possible.
John Cooper QC, representing a number of the families, told the coroner that the people making the application "are the very people who could potentially be severely criticised, and the ramifications of that are significant".
In Friday's ruling Sir John explained that Home Secretary Priti Patel had sent a PII certificate in which she stated it was her view that disclosing the material subject to her claim will damage national security.
He added she had made it clear she is aware of the importance of the public interest in disclosure of all relevant matters within the scope of the inquests and that it was also important that justice should be carried out in public.
He said: "She states that she has carried out a balancing act between these two public interests and she is satisfied that the balance is in favour of non-disclosure.
"While she has set out her conclusions, the Secretary of State accepts that it is ultimately for me to carry out the balancing act."
The inquests are set to examine the build-up and the attack itself, security at the arena, the emergency response and the victims and their cause of death.
They will also look at whether the attack could have been prevented and the role of the police and security services.
A similar PII certificate was submitted by Assistant Chief Constable Russ Jackson, on behalf of Counter Terrorism Police North West.
Sir John said that the risk identified by both parties was that disclosure of the information "will make it easier for terrorists to kill people by avoiding detection before they are able to carry (out) an attack".
Sir John said that Mr Cooper, on behalf of the families, had made it clear in oral submissions last week that they were interested in what information the security service and the counter-terror police had and how it affected their subsequent actions, and not with how they obtained the information.
In his ruling, he said: "I have considered in relation to each item over which PII is claimed whether it might be possible by gisting the information to minimise the risk to national security to a proportionate level. For reasons which I cannot elaborate in an open document, I am satisfied that matters are too inextricably linked to make that a realistic possibility."
The excluded evidence would likely be held in private at a public inquiry without the families of the victims and their lawyers.
Potential opportunities to stop the Manchester bombing were missed as a result of a catalogue of failings by security services, a report concluded last November.
A number of shortcomings in the handling of Salman Abedi before he launched his suicide attack were detailed by Parliament's Intelligence and Security Committee (ISC).
Abedi, 22, first came to the attention of MI5 in December 2010 and was briefly investigated by the agency in 2014.
The ISC assessment said:
- Abedi visited an extremist contact in prison on more than one occasion but no follow-up action was taken by either MI5 or police
- MI5 decided not to place travel monitoring or restrictions on Abedi, meaning he was allowed to return undetected to the UK in the days before he carried out the attack
- MI5 systems moved too slowly after Abedi's case had been flagged for review
- Abedi was not at any point considered for a referral to the Prevent anti-terror scheme
A Home Office spokesman said: "It is vital that those who survived or lost loved ones in the Manchester Arena attack get the answers they need, and that we learn the lessons whatever they may be.
"That is why it is right that an inquest is being conducted, and any decision to claim Public Interest Immunity is only taken when there is a risk of undermining our national security. The coroner's ruling today recognises that that risk applies in this case.
"If the coroner decides that an inquest cannot satisfactorily investigate the deaths the Home Secretary will carefully consider any recommendations they make."