Relatives of 33 people killed in loyalist bombings in the Irish Republic 40 years ago are suing the British government over alleged collusion.
As the anniversary of the 1974 Dublin-Monaghan bombs approaches, lawsuits have been launched against the British Ministry of Defence, chief constable of the Police Service of Northern Ireland and the Northern Ireland Secretary.
Derek Byrne, who survived an attack in the capital, and Patrick Askin, whose 44-year-old father Paddy was killed in Monaghan, are leading the damages action over the worst single day of atrocities in the Troubles.
Kevin Winters, solicitor for the group, said the lawsuit was being taken to seek discovery of documents that would support allegations of collusion between loyalist bombers and the British state.
"For want of a better description, a cover-up," he said.
Saturday May 17 is the 40th anniversary of the attacks.
Thirty-three people were killed, including a pregnant woman, and almost 300 people injured in no-warning bombs, three in Dublin and one in Monaghan in the space of 90 minutes.
The Ulster Volunteer Force was blamed.
Twenty-five people representing some of the survivors and next-of-kin of those killed in the attacks have come together for the action in the Belfast High Court in which they claim some of those involved were British agents.
Their case states that the terrorists were from Northern Ireland and many were employed by organs of the British state at the time.
Mr Winters added that the army, police and British government were responsible as they were aware of who was involved in the bombing raid but did nothing to stop them, or investigate them after.
He said: "This legal action is taken against a background of a series of previous attempts by the families to seek justice.
"It presents as the latest stage in their battle against many levels of state intransigence and indifference to their plight."
The lawsuit claims collusion, malfeasance, negligence and conspiracy to murder.
The relatives and survivors are being supported by the Justice for the Forgotten group which has campaigned for an independent inquiry into the bombings.
Spokeswoman Margaret Urwin said: "We have tried every other avenue without success and the only other option left for the families was this legal route."
The relatives have attacked the Irish Government and the Garda which they said badly let them down in their campaign.
They have also written to the Garda Commissioner asking if the force would co-operate with the Police Ombudsman of Northern Ireland investigation into the atrocities.
The letters of claim were sent at the end of April.
High Court writs will be issued when lawyers for the British state respond.
A report in 2003 by former Irish Supreme Court judge Henry Barron found grounds for suspecting the bombers may have had help from members of the British security forces but no conclusive proof.