The prosecution of Hyde Park bombing suspect John Downey has collapsed after a "reckless" error led to him being given a false assurance that he was not wanted by British police over the IRA attack, it can now be reported.
Families of the victims of the bloody 1982 attack, in which four soldiers died, said they felt "devastatingly let down" after the prosecution announced it would not appeal against the decision to throw out the case.
Convicted IRA member Downey, 62, of Co Donegal, had received a "letter of assurance" in 2007 when in fact there was an outstanding warrant against him.
Despite regularly travelling to the UK and Northern Ireland since then, in May last year he was arrested at Gatwick Airport en route to Greece and charged.
He "strenuously" denied the murder of four British soldiers and causing an explosion.
The judge, Mr Justice Sweeney, threw the case out after Downey's lawyer successfully argued that the defendant should not go on trial at the Old Bailey.
At an earlier hearing, Henry Blaxland, QC, warned of the political ramifications in Northern Ireland of pursuing a trial against Downey in such circumstances, saying the false assurance he received was "not just negligent, it was downright reckless".
In his judgment, Mr Justice Sweeney said there were "very particular circumstances" of the case.
The public interest in prosecution was "very significantly outweighed" by the public interest in ensuring that "executive misconduct does not disrepute", and in "holding officials of the state to promises they have made in the full understanding of what is involved in the bargain".
The legal dispute raises questions with the Police Service of Northern Ireland which, the court heard, knew about the UK arrest warrant for Downey but did nothing to correct the error of 2007.
Members of the victims' families were in the public gallery last Friday when the judge presented his ruling at the Old Bailey. Downey, who was sitting separately in the courtroom, declined to comment.
The final ruling could not be reported until after the prosecution announced today it would not appeal against the decision.
Reacting today, relatives of the four soldiers said in a statement: "It is with great sadness and bitter disappointment that we have received the full and detailed judgment and that a trial will now not take place.
"This news has left us all feeling devastatingly let down, even more so when the monumental blunder behind this judgement lies at the feet of the Police Service of Northern Ireland (PSNI).
"The end result is that the opportunity for the full chain of those terrible events will never be put in the public domain for justice to be seen to be done."
They continued: "Our men signed up to serve their country in good faith, yet now it seems that that faith was not supported by those within certain areas of authority.
"The families now seek a degree of accountability for this catastrophic failure."
They said that the grieving for the fallen soldiers "never stops", adding: "The torment for the families will be ongoing, knowing that John Downey will be returning to his family and life will be normal for him."
On 20 July 1982, a car bomb left in South Carriage Drive killed the soldiers as they rode through Hyde Park in central London to the changing of the guard.
The explosion killed Roy Bright, Dennis Daly, Simon Tipper and Jeffrey Young and injured other members of the Royal Household Cavalry.
Seven horses were also killed as the soldiers travelled from their barracks to Buckingham Palace.
Another horse, Sefton, survived terrible injuries and became a national hero.
The investigation into the bombing led police to Downey, through fingerprints on parking tickets and a description given by witnesses of two men carrying out reconnaissance in the area before the attack.
An arrest warrant was issued, but it was decided not to seek Downey's extradition from Ireland in 1989, in part due to the lack of strong evidence against him, the court was told.
Then in 2007, Downey received assurance he was not at risk of prosecution as part of a scheme run by the Northern Ireland police.
He was one of 187 On the Runs (OTRs) to seek clarification from the authorities in the wake of the Good Friday Agreement.
Mr Blaxland said: "Sinn Féin impresses it is impossible to overstate the importance of the assurances given to the 187 people."
Warning of the political fallout if a trial should go ahead, Mr Blaxland said: "Once the trust starts to break down the whole edifice starts to crumble."
The court heard that Downey had been heavily involved in the peace process long before the Good Friday Agreement and had even given up his oyster farm to work on it full time.
The court was told by Downey's legal team that there were other factors, aside from the clerical error, that meant Downey should not face trial.
These included the length of time - more than 30 years - since the offence, as well as the commitment in 2001 not to pursue those who might benefit from early release schemes.
However, in his judgment, Mr Sweeney only upheld the argument that the letter of assurance and the failure to correct it amounted to an abuse of process.