A group of 39 former postmasters had their convictions quashed at the UK Court of Appeal on Friday after what has been described as the country's largest collective miscarriage of justice.

A saga that has ruined hundreds of lives and sullied the image of UK's Post Office brand, its roots go back more than 20 years to the introduction of a new computer system called Horizon.

What went wrong with Horizon?

Introduced into the Post Office network in 1999, the automated computerized system Horizon – developed by Japanese company Fujitsu – was used for transactions, accounting and stocktaking. 

Within months, postmasters reported bugs in the system after seeing large financial shortfalls in their weekly stocktake. When their pleas to the Post Office were ignored, some postmasters tried to plug the gap with their own money, with some even remortgaging their homes. In many cases these efforts were not enough to fill the shortfalls.

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Assured by Fujitsu that Horizon was robust, the Post Office prosecuted 736 postmasters between 2000 and 2014. Individuals involved were told that they were the only people to have reported problems with Horizon.

Some postmasters were imprisoned following convictions for false accounting and theft. Martin Griffiths, aged 59, committed suicide after being falsely suspected of stealing $83,000. Seema Misra was pregnant with her second child when she was jailed in 2010 and was forced to give birth while wearing an offenders' electronic tag. 

Many victims have spoken of being shunned by their communities, being financially ruined, and seeing marriages break down. A number have died with convictions against their names. 

In December 2019, after a lengthy series of civil cases, the Post Office agreed to settle with 555 claimants and to pay $80m in damages, although the claimants received just $16.5m after legal fees were paid. Days later a High Court judgement said the Horizon system contained "bugs, errors and defects" and was "not remotely robust."

Following the ruling, more cases were passed to the Criminal Cases Review Commission (CCRC), an independent body investigating suspected miscarriages of justice. Until now, the CCRC has referred 51 cases back to the courts. Six people's convictions were overturned in earlier hearings, and with Friday's ruling quashing a further 39 people's convictions, the stage is set for the next stage of the campaigners' battle.

The Post Office faces a potentially enormous compensation bill. The historic shortfall scheme it set up to repay those who lost out – excluding those who were part of the High Court settlement – has received more than 2,400 claims. Although the Royal Mail (responsible for UK mail) was privatized in 2012, the Post Office (the nationwide network of postal branches) is still government-owned, meaning part of the liability may fall on the UK government.

Friday's strongly-worded ruling that the 39 convictions were "an affront to the public conscience" opens up the possibility of postmasters pursuing civil action against the Post Office for malicious prosecution, seeking significant sums in damages. A further 22 similar cases could go directly to the Court of Appeal, if a conviction is believed to be unsafe.

Solicitor Neil Hudgell, who represented 29 of the postmasters, said the Post Office was "found to have been an organization that not only turned a blind eye to the failings in its hugely expensive IT system, but positively promoted a culture of cover-up and subterfuge in the pursuit of reputation and profit."

Evidence emerged in court that Post Office staff were ordered to destroy documents in 2013 to protect the company's reputation. Postmasters were forced to plead guilty to crimes the organization knew they had not committed, the court heard.

So far, nobody at either the Post Office or Fujitsu has been held accountable, although the IT firm is being referred to the Director of Public Prosecutions for possible further action.

A non-statutory inquiry into the scandal was set up six months ago and will report in the summer. There have been calls for Prime Minister Boris Johnson to launch a larger-scale public inquiry and for Paula Vennells, who was the Post Office CEO at the time, to be stripped off her Commander of the British Empire (CBE) honor.