It comes after the case against a Co Donegal man accused of carrying out the 1982 Hyde Park bombing in London collapsed on Tuesday, after being thrown out by an Old Bailey judge.John Downey, 62, had been charged with the murders of four soldiers after his arrest at Gatwick Airport last year.He was detained because his name was on the UK's most wanted list - but in 2007, despite the outstanding warrant, he had been granted an immunity deal.Terry Spence, of the Police Federation, said he was shocked and appalled by what had happened.He went on: "This is sickening, and I know that it will re-open old wounds for scores of heartbroken families.There is a crying need for clarity as well as some honesty and decency.Terry Spence"None of us knew of the existence of this administrative scheme or what it entailed or the number of OTRs involved."I am angry, disappointed and feel betrayed. Those involved in this skulduggery should hang their heads in shame. When were they going to tell us about it, and why did it take a mistake exposed at the Old Bailey to shine a spotlight on this sorry mess?"The four members of the Royal Household Cavalry killed as they travelled to Buckingham Palace from their barracks were Roy Bright, Anthony "Denis" Daly, Simon Tipper and Jeffrey Young.In a statement, their families described their devastation at the development.Judge Mr Justice Sweeney branded the blunder by the PSNI a "catastrophic error" - while Chief Constable Matt Baggott has said that the force accepts "full responsibility" for the failures and that the matter will be referred to the Police Ombudsman.Meanwhile, a political row has developed at Stormont amid the fallout from the revelations.DUP First Minister Peter Robinson branded it "an outrage and a dark day", insisting that there should not be "a sell-by date" for justice.He said: "It is little wonder that some have lost faith in our justice system."Sinn Féin MP Francie Molloy said that his party's position that John Downey should not be prosecuted had been "vindicated".He added: "The arrest and charging by the British police of John Downey was a clear breach of commitments given by the British government at Weston Park and in subsequent negotiations."UUP Justice spokesman Tom Elliott branded the immunity assurances handed out "a disgrace".He added: "The scale of this is breath-taking. We are not talking about an isolated case, but 187 letters issued."John Downey was wanted by the Metropolitan Police in connection with a brutal and savage crime. No piece of paper should be allowed to shield him from justice."