Relatives of the 1982 Hyde Park bombing victims today won a 37-year battle for justice as the High Court ruled IRA member John Downey was responsible for the terrorist atrocity.

Four members of the Royal Household Cavalry were killed and 31 people injured when a car bomb detonated as troops made their way to the Changing of the Guards ceremony at Horse Guards.

Downey, whose fingerprints linked him to the car involved, has always denied responsible for the July 1982 attack, and a botched criminal trial five years ago was stopped before a jury had reached a verdict.

But this morning at the High Court, Mrs Justice Yip ruled that Downey had been an “active participant” in the terrorist attack and can be held responsible for the soldiers’ deaths.

"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."

Squadron Quartermaster Corporal Roy Bright, 36, Lieutenant Dennis Daly, 23, Trooper Simon Tipper, 19, and Lance Corporal Jeffrey Young, also 19, died in the bombing, which happened during a sustained period of terror attacks by the IRA.

The civil claim for damages was brought by Sarahjane Young, L/Cpl Young's daughter, against Downey in the wake of the collapsed criminal trial. She embraced fellow relatives in court after the ruling was given.

Speaking outside court, Trooper Tipper’s brother Mark said he was relieved to "get the justice those four boys deserve".

He added: "I'm a true believer in the justice system and today it hasn't failed us."

In her findings of fact, the judge said a remote-controlled bomb packed with nails and shrapnel had been put in the boot of a Morris Marina, bought at auction by an IRA plotter a week before the attack.

Thirty one people were badly injured when the bomb went off, and seven horses had to be put down as a result of their wounds.

Downey’s fingerprints were on the Portman Square and Royal Garden Hotel car park tickets where the vehicle was stored in the days before the bombing.

The judge said Downey must have been responbile for moving the vehicle between car parks and it is “probable” that he was driving on the morning of July 20, 1982 the date of the attack itself.

Downey did not attend the civil trial last week but submitted a written statement denying involvement in the attack. He has a conviction from 1974 which identifies him as an active IRA member and is currently in prison in Northern Ireland awaiting trial over a 1972 car bombing in Ireland.

The Irishman, from Co Donegal, was charged with the murders of the soldiers in the Hyde Park attack but the 2014 criminal case against him had to be abandoned mid-trial when it emerged Downey was given a written assurance from former prime minister Tony Blair's government that he was no longer wanted.

The letter was issued under the terms of the controversial On The Runs (OTRs) Scheme.

The trial judge said Downey's 2013 arrest at Gatwick Airport represented an abuse of process and he put a block on any attempts at a future criminal prosecution.

Mrs Justice Yip said an “error on the part of the state” was responsible for justice being denied in the criminal courts, as she allowed the civil claim to proceed despite being filed years after the bombing and much later than is usually permitted.

The case will return to the High Court next year for a decision on the level of damages that will be paid. Downey will also have to paid Ms Young’s legal costs.