Lawyers representing the families of the Manchester Arena attack victims have “embarrassing” evidence highlighting the “shortcomings” of the security services, a court has heard.
John Cooper QC told a pre-inquest hearing “full transparency” was required at the inquests into the deaths of the 22 people killed.
He asked the coroner not to confuse “national security” with “national or personal embarrassment”.
The inquests are due to begin in April.
Coroner Sir John Saunders said a decision on what evidence is heard in public and what is kept private would be deferred until later in the year.
Mr Cooper told the coroner documents had already been disclosed to him with “significant material, which is, if I may put it this way, of embarrassment to the security services, potentially to the police and certainly to the medical emergency services.”
He said: “National security is a very defined concept; it shouldn’t wittingly or unwittingly be used to restrain material which may further expand upon the failings of the security services, emergency services and the failings of others.
“We already have significant material which exposes shortcomings of the security services, of the emergency services and others.
“If I can put it inelegantly, the cat is already out of the bag.”
The pre-inquest review was attended by some of the families of those killed when Salman Abedi, 22, detonated a device at the end of an Ariana Grande concert on 22 May 2017.
The coroner told the hearing at Manchester Town Hall the timetable for criminal proceedings against the bomber’s brother, Hashem Abedi, meant it was “extremely unlikely” evidence will be heard before the trial ends.
The 22-year-old has been charged with murder and attempted murder.
The court heard that Mr Abedi denies helping his brother carry out the attack.
He was extradited from Libya to the UK last week.
Sir John told the families that “as far as possible” nothing should delay the inquests, scheduled to begin on 2 April next year.
The hearing was told that the inquests will be held in a specially converted courtroom at Manchester Magistrates’ Court complex.
A further pre-inquest hearing will be held on 6 September.