Image copyright Getty Images

The families of victims of an IRA bomb attack in London's Hyde Park have held a march to seek support for a civil prosecution of the alleged bomber.

Four soldiers were killed by a nail bomb as they made their way to a Changing of the Guard ceremony at Horse Guards Parade on 20 July 1982.

The 2014 murder trial of John Downey collapsed after it emerged he had been assured he would not be tried.

Relatives now want to raise £620,000 for a civil prosecution of Mr Downey.

Roy Bright, Anthony Daly, Simon Tipper and Geoffrey Young, all from the Household Cavalry, were killed in the attack, which remains one of the most significant unsolved IRA bombings of the Troubles.

Mr Downey, who was convicted of IRA membership in the 1970s, had been Scotland Yard's prime suspect for the bombing - but was not extradited from the Republic of Ireland at the time.

Image copyright Getty Images Image caption John Downey received an assurance he would not face prosecution

In 2013, he was arrested at Gatwick Airport while en route to Greece, and was subsequently charged with the Hyde Park killings.

Mr Downey, from County Donegal, denied murder and intending to cause an explosion likely to endanger life.

However, his trial at the Old Bailey in central London was halted after it was revealed he had been given an assurance by the UK government that he was no longer wanted.

The assurance was made as part of the controversial "on the runs" scheme.

Giving his judgement on the case, Mr Justice Sweeney said Mr Downey had received an official letter in 2007 stating that he was not "wanted in Northern Ireland for arrest, questioning or charging by police".

But the Police Service of Northern Ireland later admitted making a mistake by also assuring Mr Downey it was not "aware of any interest in you by any other police force", when he remained a suspect at Scotland Yard.

The families of those killed in the bombing are now seeking the money to pay for a civil prosecution of Mr Downey, and held a march in London on Saturday to mark the 35th anniversary of the attack.

Image copyright PA

Supporters marched from barracks in Kensington to Wellington Arch in Hyde Park.

Mark Tipper, whose teenage brother Trooper Simon Tipper was among those killed, said his families had never been given the chance to find out who carried out the attack.

"My brother had only been married a week. It was his first duty back after honeymoon, he was just 19. Nobody could imagine what that poor bride felt like, and 35 years on I know she still hurts," he said.

"If that letter had not have gone to Downey that trial would have continued and he would have been found either guilty or not guilty.

"We as families were never given that chance. All we want to know is that if this man did it, he will be brought to justice."

Who are the 'on the runs'?

They are about 187 former IRA members who had been wanted by police during the Troubles, between 1969 and 1998.

The signing of the 1998 Good Friday Agreement meant anyone convicted of paramilitary crimes was eligible for early release - but it did not cover those suspected of crimes or those who had escaped from prison.

Negotiations continued between Sinn Féin and the government over how to deal with those known as "on the runs" after the signing of the agreement.

The UK government agreed in 2001 to give assurances that there would be no prosecutions of on the runs if paramilitary organisations continued to support the peace process.

Initially, the government tried pass laws to ensure they would not be prosecuted for crimes for which they were wanted.

But after facing opposition, government negotiators offered letters saying they were no longer wanted by police

Details of letters sent to on the run only became more widely known in February 2014, following the collapse of Mr Downey's trial.

On the Runs - key questions

History of the on the runs