A failure by police to fully disclose documents in a major legal action over alleged collusion with loyalist terrorists behind the Miami Showband massacre is "appalling", a High Court judge has said.

Expressing anger at the ongoing delay in providing all material to lawyers representing survivors and relatives of murdered group members, Mr Justice Maguire warned he may consider striking out the PSNI's defence to the claim.

He said: "This is an appalling situation where this case has been going on since 2012, and we are at stage in 2020 where the obligation of discovery on the police service has not been complied with.

"The court seems to be getting the runaround. It makes me angry (and) shows so much disrespect to the court."

Read More: MoD documents link Robert Nairac to Miami Showband massacre

Victims of the atrocity are suing the Ministry of Defence and PSNI over the suspected level of collaboration between serving soldiers and the paramilitary killers.

Three members of the popular band were taken from their tour bus and shot dead on a country road after a gig in Banbridge, County Down in July 1975.

They were travelling home to Dublin when a fake army patrol made up of UDR soldiers and UVF members stopped them at a bogus checkpoint outside Newry.

Band members were made to line up at the side of the road while attempts were made to hide a bomb on the bus.

The device exploded prematurely, killing some of the would-be bombers.

Gunmen then opened fire on the band, murdering lead singer Fran O'Toole, guitarist Tony Geraghty and trumpeter Brian McCoy.

Two other band members, Des McAlea and Stephen Travers, were also injured but survived the atrocity.

In 2011 a report by the Historical Enquiries Team raised collusion concerns around the involvement of an RUC Special Branch agent.

It found that UVF boss Robin Jackson, a one-time UDR member who died in 1998, had been linked to one of the murder weapons by fingerprints.

Jackson claimed in police interviews he had been tipped off by a senior RUC officer to lie low after the killings.

He went on trial charged with possession of a silencer attached to a pistol used in the murders but was subsequently acquitted.

Two serving members of the UDR were, however, eventually convicted for their part in the attack.

Based on documents uncovered by campaign groups, writs have been issued against both the MoD and chief constable.

Damages are being sought for assault, trespass, conspiracy to injure, negligence and misfeasance in public office.

Military chiefs allegedly knew about but failed to stop loyalists infiltrating the UDR's ranks, according to the victims' case.

They also claim police are liable for vetting carried out on applications to join the army regiment and the use of agents such as Jackson.

Army files have now been disclosed to the plaintiffs as part of the discovery process.

Although some of the documents have been redacted based on issues of national security, the material reportedly contains a link to undercover British soldier Robert Nairac

Captain Nairac was abducted and killed by the IRA in 1977 and his body has never been found.

His alleged connection to the case was not mentioned at a hearing yesterday dealing with the level of discovery.

Instead, counsel for the victims and bereaved contended that the PSNI's obligations remain incomplete.

Granting a two-week adjournment, Mr Justice Maguire said: "I think the time is coming in this case where the court's patience is (running out)."