A man accused of the Hyde Park bombing will not be prosecuted because he was mistakenly given a guarantee he would not face a criminal court.

John Downey had been charged with the 1982 IRA attack in which four members of the Household Cavalry and seven of their horses were killed.

He had been wanted by the Metropolitan Police for more than 30 years, but officers were unaware Mr Downey and dozens of other terrorist suspects had been promised they would not be arrested because of the Northern Ireland peace process.

Mr Justice Sweeney made his ruling at the Old Bailey last Friday, but it is only now that his decision can be reported because the Crown Prosecution Service has decided not to appeal.

The judge said it was a rare case which "offends the court's sense of justice and propriety to be asked to try the defendant".

He said even allowing for the feelings of the victims and their families, and the desire to see someone put on trial, there was "greater public interest in holding officials of the state to promises they have made in full understanding of what is involved in the bargain".

The Hyde Park attack on July 20, 1982, provided some of the most vivid images of the IRA's bombing campaign in mainland Britain.

The Blues and Royals regiment of the Household Cavalry were making their daily procession from their barracks in Kensington, to the Changing of the Guard ceremony at Buckingham Palace.

They were accompanied by two mounted police officers.

Up to 25lbs of high explosive has been left in a parked Morris Marina on South Carriage Drive, and detonated as the parade passed.

Lieutenant Anthony Daly, aged 23, and Trooper Simon Tipper, 19, died at the scene.

Lance Corporal Jeffrey Young, 19, died the next day and 36-year-old Squadron Quartermaster Corporal Roy Bright, died two days after that.

Some 31 other people, including tourists who were watching the parade, were injured.

Images of dead horses strewn across the park shocked Britain almost as much as the human casualties.

One of the horses which survived, Sefton, became a national celebrity as he recovered from his injuries.

Mr Downey was identified as a suspect within weeks of the bombing.

The police believe they found a fingerprint match on two car park tickets issued for the vehicle used in the bombing.

But having reviewed the evidence in 1989, the-then Attorney General Sir Patrick Mayhew decided it was insufficient to seek Mr Downey's extradition from the Irish government.

Then came the peace process, the permanent IRA ceasefire, and a deal which permitted those convicted of terrorist offences to be freed from jail.

But the issue of suspects deemed to be On The Run (OTR), who were wanted for questioning but never convicted, remained in limbo.

Tony Blair's government tried to find a way of legislating an amnesty.

In December 2006 he sent a confidential letter to Sinn Fein President Gerry Adams.

"I have always believed that the position of these OTRs is an anomaly which needs to be addressed. Before I leave office I am committed to finding a scheme which will resolve all the remaining cases."

But despite extensive talks the politicians could not agree on legislation.

Instead reassuring letters were sent to those involved - including Mr Downey.

On the 25th anniversary of the bombing he received this letter from the Northern Ireland office.

"The Secretary of State for Northern Ireland has been informed by the Attorney General that on the basis of the information currently available, there is no outstanding direction for prosecution in Northern Ireland.

"There are no warrants in existence nor are you wanted in Northern Ireland for arrest, questioning, or charge by the police.

"The Police Service of Northern Ireland are not aware of any interest in you from any other police force in the United Kingdom."

But the letter was wrong. The Metropolitan Police still had Mr Downey's name on the Police National Computer in connection with the Hyde Park murders.

And although he had visited both Northern Ireland and mainland Britain on numerous occasions from 2009 to 2013, when he made a brief stopover at Gatwick Airport in May last year on his way to Greece, he was arrested and later charged.

The trial at the Old Bailey was due to get under way in January, but Mr Downey's lawyers attempted to get the case thrown out because of abuse of process.

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