John Downey was an “active participant” in the IRA bombing at Hyde Park in 1982, the High Court has ruled.
It paves the way for a damages claim to be made against him.
The attack killed four soldiers from the Household Cavalry and injured 31 people.
Mrs Justice Yip told the court she was satisfied Mr Downey was “an active participant” in a carefully planned attack.
The judgement was made in London following a civil action brought by one the victim’s families.
It followed the collapse of Mr Downey’s murder trial five years ago, when it emerged he had a guarantee against prosecution issued by the government, known as an on-the-run letter.
Announcing her conclusions in London, the judge said: “This was a deliberate, carefully planned attack on members of the military.
“I have found that the defendant was an active participant in the concerted plan to detonate the bomb, with the intent to kill or at least to cause serious harm to members of the Household Cavalry.”
During evidence, the court heard 67-year-old Mr Downey’s fingerprints were found on two car park tickets connected to the vehicle used in the attack.
They were handed in at payment booths in the days and hours leading up to the bombing on 20 July 1982.
The bomb was packed with nails and was detonated by remote control as Household Cavalry soldiers made their way to the changing of the guard in Whitehall.
Mr Downey was convicted of IRA membership by an Irish court in the 1970s.
He is currently remanded in custody in Northern Ireland charged with the murders of two Ulster Defence Regiment (UDR) soldiers in Enniskillen in 1972.
The attack killed Squadron Quartermaster Cpl Roy Bright, Lt Anthony Daly, Trooper Simon Tipper and L/Cpl Jeffrey Young.
Lawyers acting for Sarah-Jane Young, L/Cpl Young’s daughter, in whose name the action against Downey has been brought, told a hearing in London last week that the families of those killed expected justice to be done.
Mr Downey, from County Donegal in the Republic of Ireland, did not play any part in the trial but filed a written defence denying any involvement in the attack.
The case will now progress to a second stage to determine the amount of damages to be awarded.
Who are the “on-the-runs”?
The Northern Ireland Good Friday Agreement of 1998 meant anyone convicted of paramilitary crimes was eligible for early release.
However, that did not cover those suspected of such crimes, nor did it cover people who had been charged or convicted but who had escaped from prison.
Negotiations continued after the signing of the agreement between Sinn Féin and the government over how to deal with those known as “on-the-runs”.
Sinn Féin sought a scheme that would allow escaped prisoners and those who were concerned they might be arrested to return to the UK but a formal legal solution proved difficult to establish in the face of strong unionist opposition.
Against that backdrop, the IRA had still not put its weapons beyond use and Sinn Féin needed grassroots republicans to continue supporting the peace process.