Scotland's official prosecutors were last night facing accusations of deep incompetence after the collapse of the World's End trial resulted in the acquittal of a serial sex killer.

Angus Sinclair, 62, who has already been convicted of two killings and a string of serious indecent offences, was yesterday cleared of murdering Helen Scott and Christine Eadie, both 17, after a judge ruled there was insufficient evidence against him.

But as politicians lined up to question the Crown Office's handling of the World's End case, particularly its decisions to lead so little available evidence, there were also allegations from senior police figures in Strathclyde that a tactical decision meant four families in Glasgow had also been denied full justice.

They had wanted the Crown to charge Sinclair with the murders of Anna Kenny, Hilda McAuley, and Agnes Cooney, who all died similar deaths to Helen and Christine in 1977, and they also believed he could be held responsible for killing Frances Barker earlier that year.

The Strathclyde Police view was based on an FBI profiler who said the same man had carried out all six murders. However, senior officers claim, the Crown had already secured a conviction for the death of Ms Barker, and was unwilling to risk a claim for miscarriage of justice, deciding instead to separate off the World's End deaths, for which the Crown had stronger forensic evidence.

One Strathclyde officer said last night: "What they feared was this would have exposed the Frances Barker case as a miscarriage of justice. Following on from Shirley McKie and Lockerbie, there was no appetite for another. I was convinced Sinclair would walk unless they included all the cases together."

Another senior officer involved in the World's End case, said: "It was the Crown that decided it did not want to proceed at this time with the Glasgow cases. There is no forensic evidence in the Glasgow cases, so there is now no way they could be prosecuted.

"If they had been rolled up together then it would have been different, but the Crown was keen to go for what it saw as a safe option."

Concern about the Crown's tactics was heightened by the mysterious disappearance yesterday morning of the man presenting the case against Sinclair.

Advocate-depute Alan Mackay was not in court when Lord Clarke threw out the charges, upholding a defence motion after just eight days of evidence in a case scheduled to last six weeks.

The judge ruled that the prosecution had failed to put forward enough evidence linking Sinclair to the murders, and agreed with defence counsel Edgar Prais, QC, that the Crown had failed to prove the girls had been abducted, raped, and robbed before they were murdered.

Mr Mackay was said to have been "depressed" by the decision and took a train to England before contacting his wife later from an unknown location, saying he was safe but "needed some time". Lothian and Borders Police said they were treating him as a missing person and were trying to trace the prosecutor.

Politicians called for an inquiry into the collapse of the trial. Margo MacDonald, Independent Lothian MSP, has lodged a question with the First Minister, asking whether an inquiry will be heard into Sinclair's acquittal.

She said: "It raises disturbing questions about the way in which the prosecution office worked. I understand they led less than a fifth of the evidence they had. It appears as though somebody has slipped up somewhere."

Margaret Curran, Labour's Scotland justice spokeswoman, said it was "deeply disappointing" the trial had been thrown out due to lack of evidence. Her Tory counterpart, Bill Aitken, added it was right that questions would be asked in the days to come.

The father of Helen Scott said he still believed Sinclair was involved in his daughter's death. Speaking outside the High Court in Edinburgh, Morain Scott, 77, said: "I am absolutely shattered."

Jim Reehill, whose sister Catherine was killed by Sinclair in 1961, said: "I'm just glad he is locked up and can't hurt anyone else."

Lothian and Borders Police said they had no plans to reopen the investigation. Sinclair was accused of acting alongside his brother-in-law, Gordon Hamilton, who has since died, in the girls' rape and murder.

A police spokesman said: "We put together a thorough and detailed case for the Crown Office and today's announcement is disappointing. Our primary concerns are for the families of Christine Eadie and Helen Scott. No-one can imagine the torment they have been put through over the past 30 years."

A spokeswoman for the Crown Office said: "Following a thorough and lengthy investigation conducted by the police working closely with the Crown Office and Procurator Fiscal Service, the Crown considered there was sufficient evidence to indict Angus Sinclair for the appalling murders of Helen Scott and Christine Eadie.

"In all trials, the prosecutor has a duty continually to consider and review the available evidence with a view to deciding how best to proceed with the trial. The Crown was of the view there was sufficient evidence on which to base a prosecution. There is no right of appeal against the acquittal by the judge."

Tom Woods, the former deputy chief constable of Lothian and Borders Police and a leading officer in the case, said: "I am bitterly disappointed for the families of Helen and Christine but I am content that over the 30 years Lothian and Borders Police have done their utmost and have conducted themselves with the highest professional standards. We did all we could."