Families of soldiers killed in the IRA bombing in London's Hyde Park said they have been made victims "three times over" in the years since the atrocity.
Relatives of some of the members of the British army's Royal Household Cavalry who died in the 1982 attack have launched a campaign as they try to raise funds to bring a legal case.
Mark Tipper, whose 19-year-old brother died in the blast, blamed former UK Labour prime minister Tony Blair for a "shameful and secret deal" which allowed convicted IRA member John Downey to walk free.
Under the controversial on the runs scheme (OTRs) - allegedly agreed by Mr Blair's Labour government and Sinn Féin - individuals were told they were not being sought by the authorities.
"The chief suspect walked free from the Old Bailey because of a shameful and secret deal by Tony Blair with the IRA," Mr Tipper said.
"Because of this we have been forced to take our own private legal action."
With the Legal Aid Agency refusing "again and again" to fund the case, Mr Tipper added: "We have been made victims three times over.
"First by the bomb, then by Blair and now by the bureaucrats of the Legal Aid Agency."
Downey, from Co Donegal, was charged four years ago over the Hyde Park bombing, which he denied, but the prosecution at his trial in the Old Bailey collapsed in 2014.
The case against him was ended because British government officials mistakenly sent him a letter in 2007, as part of the OTRs scheme, telling him he was no longer a wanted man.
Mr Tipper said the Government does not do enough to support victims of terrorism.
He said they have been "blocked at every turn" in their fight for justice and that all they are seeking is the "truth about who murdered our loved ones".
"The only way to do that is to bring the chief suspect to court, a man who has been linked by police files to six separate terrorist attacks," Mr Tipper said.
He also called on Prime Minister Theresa May to meet with the families, and to "step in" and "do the right thing".
He said: "It's in her power, not just to help us in our fight for justice, but to do better for all victims and veterans who served this country.
"We hope she will succeed where her predecessors have failed."