A convicted IRA member in jail on separate murder charges was an “active participant” in the 1982 Hyde Park bombing, the high court has ruled in a civil action taken by the victims’ families.

Relatives of four soldiers who died in the IRA blast were told that John Downey was liable for the explosion that killed their loved ones.

The families said the ruling meant those who thought they would get away with Troubles-related crimes should think again.

The civil case against Downey was brought by Sarah-Jane Young, the daughter of L/Cpl Jeffrey Young. He, along with Squadron Quartermaster Cpl Roy Bright, 36, Lieut Dennis Daly, 23, and Trooper Simon Tipper, 19, died in the IRA attack.

Thirty-one others were injured and seven horses had to be put down as a result of their wounds.

Downey denied in written evidence he had any involvement in the Hyde Park bombing.

The civil case relating to the atrocity was taken in response to the collapse in 2014 of the original criminal trial against Downey after a “letter of assurance” was produced in court. It stated that Downey had been told by the Police Service of Northern Ireland (PSNI) he was no longer wanted for any crimes related to the Troubles. The judge in the first trial concluded therefore that Downey’s arrest at Gatwick airport in 2013 was an abuse of process.

Downey’s “comfort letter” was part of a secret scheme drawn up by Tony Blair’s government to assure IRA fugitives they were no longer being sought for arrest and conviction over crimes committed prior to the 1998 Good Friday agreement.

The families of the victims’ legal team told the high court there had been clear evidence of Downey’s involvement in the attack, including “damning” fingerprint evidence. Lord Brennan QC described the IRA attack in 1981 as “cold-blooded killing, with vicious brutality and maximum harm”.

Mrs Justice Yip ruled that Downey was liable for the bombing. Wednesday’s judgment opens the way for the families to obtain damages and also, possibly to sue Downey. Their civil action was modelled on the case taken by families of the victims of the Real IRA 1998 massacre in Omagh.

No one has been convicted directly with killing 29 people in the Co Tyrone town 21 years ago but families won the right in the high court to sue several men said in court to be Real IRA leaders at the time of that atrocity.

In a statement issued through their lawyer, Matthew Jury, of McCue & Partners, the families of the Hyde Park Justice Campaign said: “Yesterday, many responsible for the most awful acts of terrorism on British soil were living out their days in peaceful retirement believing they would never be held account for their crimes.

“Today, the forgotten victims are finally remembered. Sarah Jane and the brave families gathered here wish to tell them that no matter how many years have passed justice can and will and must be done. And they invite the British government to meet with them to ensure it never fails them or other victims or veterans again.”

Downey’s legal team originally cited an official “On the run” letter he had received in 2007, which stated he was not wanted by police.

His case highlighted the highly controversial practice of “On the runs” or “Get-out-of-jail” cards given in secret by Blair’s government to Sinn Fein for more than 180 wanted IRA fugitives.

After Downey walked free, the then prime minister David Cameron ordered an inquiry into the scheme by Lady Justice Hallet. She later ruled that the PSNI had made a “catastrophic mistake” in advising the court in the original trial over Downey’s letter. Hallet found that the letters to IRA fugitives were not an amnesty and that future prosecutions of those who held the cards could go ahead.

Downey is being held at Maghaberry maximum security prison in Northern Ireland facing separate charges.

The 67-year-old from Creeslough in County Donegal in the Irish Republic is accused of murdering Ulster Defence Regiment (UDR) soldiers Alfred Johnston and James Eames in an IRA bomb attack in Enniskillen in Co Fermanagh in 1972.