One of the longest sagas in the history of criminal justice was concluded yesterday when the court of appeal quashed the convictions of Michael McMahon and David Cooper for the 1969 Luton post office murder.

McMahon and Cooper, both of whom have since died, were convicted in 1970 of the murder of Reginald Stevens, a sub-postmaster who was shot dead in September 1969 during a bungled robbery by a four-man gang. A third man, Patrick Murphy, was also convicted, but released on appeal in 1973. He too has since died.

The case became a cause celebre - and a constitutional shuttlecock between the executive and the judiciary, as successive home secretaries referred the case to appeal, only for senior judges to dismiss the appeals. This happened five times in the 1970s.

In 1980, Sir Ludovic Kennedy published a book about the case, Wicked Beyond Belief. Three weeks later, McMahon and Cooper, whose real name was John Disher, were released from prison by special order of the home secretary, William Whitelaw. Their convictions for murder, however, remained on the record.

Yesterday, Lord Justice Kennedy, sitting with Mr Justice Cresswell and Mr Justice Bennett, effectively conceded that the politicians had been right. "There are now a number of matters which can be described as causes for genuine concern," said Lord Kennedy.

"For present purposes it is unnecessary to say that any one of those matters is decisive. It is sufficient to say that, in their totality, they persuade us that these convictions are no longer safe."

Two important aspects were the court's inconsistency in allowing Murphy's appeal but refusing those of McMahon and Cooper, when all three were convicted on broadly the same evidence; and the conduct of the senior investigating officer, Detective Chief Superintendent Kenneth Drury.

Along with two career criminals, Alfred Matthews and Michael Good, Drury was central to the case. Stevens was shot in daylight in a public car park, so there were many witnesses. Almost straight away, the police received information that the murder weapon belonged to Good, and that Matthews, who had driven to Luton from the East End of London in his own car, was one of the gang.

Drury, who has since died, allowed Matthews to admit to a very minor role and turn Queen's evidence, giving testimony in court against McMahon, Cooper and Murphy. Good was arrested but never charged. Drury even ensured that Matthews and Good received the lion's share of the post office reward money.

In a statement, one officer who worked with Drury as a "close friend and colleague and fellow freemason" said he often received payoffs from criminals for not prosecuting them. He said Drury was "a past master of the arts" of "falsifying or manipulating alibi statements", "the manipulation of identification procedures" and "the repeated harassment of witnesses until we had got what we wanted from them".

In July 1977, Drury was convicted of five counts of corruption in relation to other matters, and jailed for eight years.

Yesterday Sue McMahon, McMahon's widow and his girlfriend at the time of his arrest in 1969, said: "It's a really good result, but I don't feel elated, just quite flat. It's been 34 years, and it's such a shame that Mick and John aren't here."

Cooper died in 1993 and McMahon on his 55th birthday in 1999. The convictions were quashed on Sue McMahon's 55th birthday.

Mrs McMahon and Cooper's brother Terry Disher believe both men would be alive today were it not for the case. "Without a doubt," Mr Disher said. "It affected him mentally and physically. He came out of prison a changed man and couldn't survive on the outside."

The solicitor Gareth Peirce took on the case in 1974, and it was her submission to the criminal cases review commission that led to the sixth and final appeal.

"There were many other miscarriage of justice cases in the 70s and 80s in which the court took its approach from what had happened in this case," she said. "To think of all those wasted lives; it's tragic and cruel and wrong beyond belief."

Sir Ludovic, who attended the hearing, said: "I feel delighted, but it should have happened years ago."

September 10 1969 Reginald Stevens shot dead in Luton

March 19 1970 McMahon, Cooper and Murphy found guilty at the Old Bailey, and sentenced to life, with a recommended 20-year term

February 26 1971 Case goes to appeal: dismissed

November 13 1973 Second appeal; Murphy's conviction quashed

February 12 1975 Third appeal: dismissed

July 22 1976 Fourth appeal: dismissed

July 7 1977 Kenneth Drury, senior investigating officer, jailed for eight years for corruption

April 11 1978 Fifth appeal hearing; new evidence rejected

June 26 1980 Sir Ludovic Kennedy's book, Wicked Beyond Belief, published

July 18 1980 Cooper and McMahon are released from prison by order of the home secretary, William Whitelaw

September 1993 Death of David Cooper (real name John Disher)

June 25 1999 Death of Michael McMahon

June 30 2003 Sixth appeal hearing opens

July 31 2003 Convictions of Cooper and McMahon quashed

One of the country's most contentious miscarriage of justice cases was referred to the appeal court yesterday, raising hopes that posthumous pardons will be awarded to two men convicted of murder.

The decision gives another remarkable twist to a case known as the Luton post office murder, and may clear the names of David Cooper and Michael McMahon, following a campaign which has lasted 30 years.

Despite fresh evidence that seemed to show both men had been wrongly convicted, the court of appeal had consistently refused to set them free in the 1970s. There had been five hearings.

In 1980, the home secretary, then William Whitelaw, took the extraordinary step of ordering both men released from jail, but he did not quash their convictions.

Cooper died in September, 1993, and McMahon in July 1999, still protesting their innocence. Their families believe the trauma of their wrongful imprisonment led to their relatively early deaths.

The case concerned the death of the sub-postmaster Reginald Stevens, who was shot dead in a bungled robbery in car park near the Luton post office on September 10, 1969. Stevens was killed when he refused to hand over his keys.

The four-man gang, who were using their own cars, threw the murder weapon over a railway embankment and made their getaway south down the M1. A witness took down one of the car numbers. This sighting led police to Alfred Matthews, a north London villain already convicted for a post office robbery. The gun was owned by Michael Good. Both men fitted descriptions given by witnesses. Neither of them went to jail, and, on police recommendation, each got substantial shares of the reward money put up by the GPO.

The man in charge of the investigation, Detective Chief Superintendent Kenneth Drury, did not arrest Good, and he allowed Matthews to become a prosecution witness against Cooper, McMahon and another man, Patrick Murphy.

At their trial at the Old Bailey in March 1970, the judge, Mr Justice Cusack, told the jury that Matthews would have to be "wicked beyond belief" to have invented the participation of three innocent men. The jury convicted the three and they were sentenced to life imprisonment, with a recommendation that they serve at least 20 years.

The case became a cause célèbre as witnesses for Cooper, McMahon and Murphy came forward. There were also claims that vital evidence was withheld from the defence. Drury was subsequently jailed for corruption.

The case was heard by the court of appeal in 1971, in 1973 - when Murphy's conviction was quashed - and in 1975, 1976 and 1978.

Although there were profound doubts over the guilt of Cooper and McMahon, the court refused to free them, leading Gareth Peirce, their lawyer, to say the case had become "a battle between the judiciary and the executive." Lord Devlin, probably the most widely respected judge of his day, declared: "Doubts do not just lurk; from the first they have flown about the case like bats in a belfry."

Sir Ludovic Kennedy, the champion of wrongful convictions, in 1980 published the book, Wicked Beyond Belief, which contained edited extracts of prison journals written by Cooper and McMahon.

Less than a month later, Whitelaw, a friend of Sir Ludovic, ordered the men's release from prison.

Sir Ludovic revealed in a later book, On My Way to the Club, that Whitelaw had wanted to pardon the men, but had been prevented from doing so by the lord chief justice of the time, Lord Lane.

Cooper and McMahon never came to terms with the unsatisfactory nature of their release.

In January 1993, Cooper, whose real name was John Disher, was found wandering in a dazed state with a huge gash at the back of his head. How he was injured was not established, but he never recovered and died in September of that year, aged 51.

At the time McMahon told the press: "I look at this case and see that everyone is dying. The trial judge is dead, Drury is dead, Lord Devlin is dead, and now poor John is dead. I have to ask myself, is this what they want? Do they want me to die too, so the whole wretched affair just fades away?"

McMahon died of a heart attack on his 55th birthday, on June 25, 1999. Since then, lawyers have found fresh evidence about Matthews's and Good's roles as police informers, and the work of Drury.

Sue McMahon, Michael's widow, said yesterday: "I'm very pleased the case has been referred back, but the fact that John and Mick are not here, that's the saddest part."

One of Britain's longest running alleged miscarriages of justice, in which two men were convicted of murdering a postmaster more 30 years ago, was referred to the Court of Appeal for a record sixth time yesterday.

Both men, David Cooper and Michael McMahon, who were sentenced to life imprisonment for the "Luton post office murder" in 1970, died while trying to clear their names.

The latest appeal is based on the discredited evidence of the senior police officer in the case who was later jailed for taking bribes. The evidence from a key informer, who was paid for his information, is also now considered unreliable. The Criminal Cases Review Commission, which has examined the new developments, referred the case to the Court of Appeal.

The murder was committed on the evening of 10 September 1969. Reginald Stevens locked up his post office and headed for a nearby car park. There, waiting for him, was a gang of robbers. In their attempts to force him to hand over the post office keys, one of the gang shot him in the stomach, killing him instantly. Three men were seen fleeing the scene in a green van. Later, others saw the men transfer into a blue Vauxhall and a red sports car. One witness noted the cars' registration numbers.

The discovery of the discarded shotgun, the witnesses, and one of the number plates led police swiftly to a man known as Alfred "the Fox" Mathews. But, instead of facing a murder charge, he ended up receiving a £2,000 reward from the Post Office.

He was paid for identifying his three alleged accomplices – Cooper, McMahon and Patrick Murphy. The man who named Mathews for the reward money was Detective Chief Superintendent Ken Drury, the officer in charge of the inquiry.

Since the conviction of Cooper and McMahon, a former police officer has come forward to contradict the evidence given at the men's trial by Ch Supt Drury, who was later jailed for taking bribes from a pornography baron in London. New evidence has also emerged to suggest that Drury – now dead – took a share of the original Post Office reward money.

The three men named by Mathews all had alibis, however, showing that they were nowhere near Luton at the time of the shooting. But Drury found two convicted prisoners to testify that they heard McMahon confessing to the crime while on remand.

The judge, Mr Justice Cusack, in his summing up, said that while there was virtually no evidence other than Mathews' testimony, it would be "wicked beyond belief" for Mathews to involve three innocent men. He sentenced all three to life imprisonment, with a recommendation that they serve at least 20 years.

Cooper and McMahon each served 11 years in prison. In 1980, William Whitelaw, who was Home Secretary, took the extremely unusual step of ordering their immediate release, using the prerogative of mercy.

But their release was not accompanied by a pardon. Their names were never formally cleared, and Cooper's life was destroyed. After his release he became semi-reclusive and developed a drink problem.

In January 1993 he was found lying in the road with a gash at the back of his head. It is not known if he was struck or simply fell. He died in July 1995 and McMahon died in July 1999.

Only Murphy was able to clear his name, proving with new evidence that he could not have been in Luton at the time of the murder and that Mathews had lied about his involvement. But, despite repeated referrals from successive home secretaries, appeal judges have refused to accept new evidence for McMahon and Cooper.