The High Court has ruled that a convicted IRA member was an “active participant” in the 1982 Hyde Park bombing after a case was brought by victims’ families.

Relatives of the four soldiers who died in the blast launched the civil action against John Downey after a criminal case collapsed.

Mrs Justice Yip ruled on Wednesday that he was liable for the bombing, paving the way for damages to be awarded.

“This was a deliberate, carefully planned attack on members of the military,” she said.

“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.”

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The case will now progress to a second stage to determine the amount of damages to be awarded.

Mark Tipper, whose brother Trooper Simon Tipper was killed in the bombing, said he was relieved to “get the justice those four boys deserve”.

Speaking outside the Royal Courts of Justice, he added: “I’m a true believer in the justice system and today it hasn’t failed us.”

Mr Tipper said the ruling sent an important message to “any other terrorist out there who thinks he can get away with it”.

The families’ lawyer had told the High Court that there was clear evidence of Downey’s involvement in the attack, including “damning” fingerprint evidence.

Lord Brennan QC called the bombing a “cold-blooded killing, with vicious brutality and maximum harm”.

“Thirty-seven years after, if justice can properly be done, as it can be in this case, then let it be done,” he added.

“That will reflect the expectation of the bereaved families and the injured, it reflects the state of our law, it accords with the conviction of our community and the sentiment lying beneath it is deeply felt.”

The case has been brought by Sarah-Jane Young, the daughter of Lance Corporal Jeffrey Young.

He was 19 when he was killed by a car bomb as he rode through Hyde Park to attend the changing of the guard with the Royal Household Cavalry on 20 July 1982.

The flag-draped coffin carrying Lt Dennis Daly, the Blues and Royals officer killed in the Hyde Park bombing (PA)

The other victims were Squadron Quartermaster Corporal Roy Bright, 36, Lieutenant Dennis Daly, 23, and Trooper Simon Tipper, 19, while 31 others were injured and seven horses had to be put down.

The relatives initially asked for help with legal fees through crowdfunding after being refused legal aid five times, but it was revealed in February last year that they had been granted public funding.

Downey filed a written defence denying any involvement in the attack, which came on the same day a second bomb was detonated at a military concert in Regent’s Park, killing six bandsmen.

He is currently in prison in Northern Ireland, facing a criminal prosecution for another car bomb attack which killed Ulster Defence Regiment members Alfred Johnston and James Eames in Enniskillen in 1972.

Downey was initially charged with murder over the Hyde Park bombing but his trial at the Old Bailey collapsed in 2014 in controversial circumstances.

A judge threw out the case after his defence lawyer presented a 2007 “letter of assurance” from the Police Service of Northern Ireland (PSNI) that he was not at risk of prosecution.

The letter was issued under the terms of the controversial On The Runs (OTRs) scheme, which saw more than 180 Irish republicans suspected of involvement in terrorism assured that they would not be prosecuted.

When Downey received his letter he was in fact wanted by London’s Metropolitan Police, and the communication was a mistake.

But trial judge Mr Justice Sweeney ruled that Downey’s arrest at Gatwick airport, as he passed through the UK on the way to a holiday in 2013, represented an abuse of process and he put a stay on any future prosecution.

Ms Young’s solicitor, Matt Jury, said she and other relatives had been made victims “three times over” by the bombing, the OTR letter and the trial collapse.​

“They, and thousand whose lives were devastated by the IRA, are the forgotten victims,” he added.

“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 to account for their crimes. But justice has prevailed.”