The Historical Enquiries Team (HET) review of the police investigation into the McGurk's Bar massacre was completed last December.
But the release of the findings have been blocked by Chief Constable Matt Baggott, according to relatives of the 15 people blown up when the north Belfast pub was hit by the loyalist UVF in December 1971.
They have now instructed solicitors to go to the High Court in an attempt to obtain a copy of the report.
They have also lodged a complaint with the Northern Ireland Police Ombudsman Michael Maguire. A second written complaint was handed over to the Policing Board on Thursday.
The families said they made repeated requests to the HET, the Policing Board and the Chief Constable and were reluctantly left with no choice but to instruct their solicitors, Kevin Winters & Co. to begin a legal action.
At the time of the attack, security sources lied when they claimed the bomb went off accidentally as it was being handled by the IRA.
Just one man was convicted and sentenced to life imprisonment in 1977 for his part in the attack.
Ciarán MacAirt, grandson of Kathleen Irvine, one of the 15 Catholics murdered in the no-warning explosion, said: "The state and its security forces conspired to criminalise our loved ones when they fabricated a story that the bomb was in-transit and that customers were being schooled in bomb-making."
To clear their good names, our families have been forced to campaign against police lies and intransigence for two generations. We have done so constitutionally and with great dignity.
"By blocking the release of the HET report into the death of our loved ones, the Chief Constable of the reformed Police Service of Northern Ireland is quite simply re-traumatising our family members. Many of them are aged," he added.
"Despite our great patience and despite our facilitation of the HET's investigation, we have been forced to begin legal proceedings to gain access to this report."
He continued: "We have been forced to raise a complaint with the Police Ombudsman and with the Policing Board, set up as they are to hold the police to account. Our families have suffered enough."
A spokesperson for the PSNI said: "It would be inappropriate for us to comment because the Ombudsman is involved."
The HET, a special investigative unit set up to review unsolved murders in Northern Ireland, started investigating the McGurk's atrocity in March 2006.
Mr Baggott is already under pressure from the bereaved relatives after failing to unequivocally endorse the findings of a separate Police Ombudsman's inquiry into the bombing which claimed the RUC had an "investigative bias" that misled people into wrongly thinking the IRA was responsible, when in fact the device was planted by loyalists.