The head of Northern Ireland's prosecution service tonight defended his decision to put Chris Ward in the dock over the £26.5 million Northern Bank robbery. Alasdair Frazer's office came under heavy criticism yesterday when the case against Mr Ward dramatically collapsed. The Public Prosecution Service (PPS) was already under pressure after trials failed to secure convictions for the 1998 Omagh bombing atrocity and the infamous 2005 republican murder of Robert McCartney. Mr Ward's solicitor yesterday said the decision to prosecute his client had been made without evidence and declared the 26-year-old's life had been ruined by the decision. But in a rare public comment on its work, the PPS tonight issued a statement which claimed it was right to take the Omagh and McCartney cases to court. It claimed the cases had met the requisite tests and had run their course, despite the failure to secure convictions. On the Northern Bank trial, the PPS said: ``The Test for Prosecution was applied with equal rigour and impartiality in the case of Christopher Ward. ``Following receipt of a complex police investigation file and having taken the advice of independent senior counsel and considered the available evidence, it was concluded that the Test for Prosecution was met. ``There is a continuing duty upon the Director in the interests of justice to keep the Test for Prosecution under review as the evidence is presented in court.'' A crucial element of the case against Mr Ward was the claim that he had been instrumental in changing the staff rota at the Northern Bank to suit the robbers' plans. But the central plank of the prosecution case was swept away when it was confirmed other members of staff played a central role in rota changes. The trial judge Justice Richard McLaughlin described the rota changes as ``a result of coincidence and chance''. The PPS tonight said: ``This part of the circumstantial case was of fundamental importance. Evidence in relation to the rota was given. ``There were differences between the evidence referred to by senior counsel in his opening and the evidence which in fact was given by the witnesses during the course of the trial.'' The prosecution service said the Director then decided that the case could not proceed. In a summary of the task facing the prosecution service, the PPS underlined the need to weigh-up each case. ``It is impossible to know with certainty whether or not a conviction will be obtained in any particular case. What is required is that there is a reasonable prospect of a conviction on the evidence which is available,'' it said.