West Midlands Police may have been tipped off about the Birmingham pub bombings by an IRA "mole" before the deadly blasts, a coroner has been told.
It was suggested to the Birmingham and Solihull coroner Louise Hunt, hearing an application to resume an inquest into the deaths of the 21 victims in 1974, there was "reason to believe it's the case".
Ashley Underwood QC, representing some of the victims' families, also said there was suspicion among relatives that West Midlands Police's investigating officers lied to gain the wrongful convictions of the Birmingham Six.
The men were released in 1991 after their convictions for murder were overturned by the Court of Appeal.
The force's barrister, Jeremy Johnson QC, told Ms Hunt that the force's own chief constable had "no principled objection to the resumption" but claimed the coroner had no legal jurisdiction in the case.
On the night of November 21, blasts ripped through the city centre Tavern In The Town and the Mulberry Bush pubs, packed with pre-Christmas revellers.
The fatal bombings, which also left 182 people injured, are widely acknowledged to have been carried out by the IRA.
Mr Underwood said it would be a question for any future inquest as to why police may have lied to gain convictions, but suggested one such reason may be that officers wanted to cover up their informant's involvement.
He said: "These were appalling deaths and the only reason it's not been investigated, as common sense dictates, is the false convictions.
"These cry out for a proper, fearless investigation which represents the best closure these relatives can have."
Putting forward the families' application to resume the inquest, the barrister added: "There is reason to believe the gang of murderers had an informant in their ranks and that the police knew in advance.
"And there is reason to believe the police had sufficient time, between the telephone warnings and the first bomb going off, to evacuate - and that the emergency services could have arrived earlier - but that records about those things were falsified."
He recounted a 2014 newspaper interview with one of the Birmingham Six, Paddy Hill, who was also sitting in the coroners court at Solihull on Wednesday, who raised the subject of "the alleged informer".
Mr Underwood, addressing the coroner, said: "I can't say if that's true, that's for an inquest to determine.
"But if it is (true), then the police had a mole in the gang which raises the question, did they know it was going to happen and did they lie to the (criminal) court to cover their knowledge of that, and cover their mole."
He said there was a "very compelling case" to resume the inquest into the victims' deaths, in order to provide answers for the victims and their loved ones.
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Before hearing submissions, Ms Hunt had requested West Midlands Police provide her with a list of "lost" evidence, including an unexploded third bomb.
A separate police review of evidence in the case, ending in 2014, discovered that of the 168 original exhibits listed for the Birmingham Six trial, 35 items could not be located.
Opening the hearing, the coroner said: "This was a terrible atrocity resulting in the deaths of innocent people and serious injury to many more but unfortunately this hearing is going to focus very much on legal argument, so I'm sorry about that but I think it's necessary for me to make the decision that needs to be made."
The coroner will hear submissions from lawyers representing the families and the police over the next three days but will not make her decision until later this month.
Outlining the current state of the police's criminal investigation into the bombings, Mr Johnson said there had been three developments in the case including new potential forensic evidence, a book by a former IRA spymaster Kieran Conway, and a first-hand account from a now dead West Midlands fireman Alan Hill.
He told the coroner: "As and when new lines of enquiry emerge they have been and will be investigated irrespective of whether the inquest resumes.
"The chief constable (David Thompson) has no principled objection to the resumption, if there is a proper legal basis.
"But we do raise the question as to jurisdiction."
Inquests were opened and adjourned at the time, but never resumed as a result of the Birmingham Six's convictions in August 1975.
Speaking outside court before Wednesday's proceedings, Julie Hambleton - whose then 18-year-old sister Maxine was killed by the Tavern bomb - said any decision to resume a new inquest would be "seismic".
The hearing continues.