Court of Appeal quashes Post Office convictions in historic judgment
In the landmark case of R v Hamilton and others  1 Cr. App. R. 17, the Court of Appeal has quashed the convictions of 39 sub-post masters and found their prosecution by the Post Office to be an “abuse of process” and an “affront to justice”.
The main issue in this case was the reliability of the computerised accounting system, “Horizon”, which was used in branch post offices during the period 2000-2012. By recording all transactions at a branch, Horizon calculated how much cash and stock should be held in the branch. Pursuant to their contracts with the sub-postmasters, the Post Office adopted an approach which meant that “if Horizon showed a shortfall, however inexplicable to the sub-postmaster, the sub-postmaster was required to make good on the loss at the end of the trading period.” 
Prosecutions were brought by the Post Office acting as a private prosecutor and the appellants were convicted between 2003 and 2013 of offences of theft, fraud and false accounting.
The 2019 High Court Proceedings
Whilst details of Horizon’s flaws had been reported for many years, the unreliability of the system was affirmed in 2019 by the High Court in group litigation proceedings between claimants representing about 580 sub-postmasters, and Post Office Limited. In that case, Fraser J found that there were numerous bugs, errors or defects in Horizon which were capable of causing, and did in fact cause, shortfalls in post office branches. He colourfully described the Post Office’s obstinate denial of the problem as “the 21st century equivalent of maintaining that the earth is flat.” 
Due to the failure of the Post Office in disclosing the full and accurate position regarding the reliability of Horizon, the Criminal Cases Review Commission referred the appellants’ cases to the Court of Appeal. This referral gave rise to two grounds of appeal:
- The reliability of Horizon data was essential to the prosecution and, in the light of all the evidence including Fraser J’s findings in the High Court, it was not possible for the trial process to be fair;
- The evidence, together with Fraser J’s findings, shows that it was an affront to the public conscience for the appellants to face prosecution.
The Appeal Proceedings
These grounds reflected two possible circumstances in which criminal proceedings may be found to have “abused the process of the court”.
In essence, the grounds of appeal raised issues as to whether the Post Office, as a prosecutor, had properly discharged its duties of investigation and disclosure under s.3 Criminal Procedure and Investigations Act 1996 (“the CPIA”).
In each of these appeals, “the appellant relied on failures of investigation and disclosure which, it was argued, would have founded a successful application to stay the prosecution as an abuse if the relevant facts had been known at the time.” 
The Court’s Judgment
The Court of Appeal accepted Fraser J’s findings that throughout the relevant period there were significant problems with Horizon and that the Post Office knew these problems existed.
In respect of Ground 1, the Court concluded the Post Office “as prosecutor brought serious criminal charges against the sub-postmasters on the basis of Horizon data, and by its failures to discharge its clear duties it prevented them from having a fair trial on the issue of whether that data was reliable.” 
Significantly, it did not matter that many of the appellants pleaded guilty. As the Court of Appeal held in R v Togher and others  1 Cr App R 33, a conviction following a guilty plea may be quashed on grounds of abuse of process where the plea was “founded upon” the irregularity of non-disclosure. The Court was in “no doubt that all the guilty pleas of the appellants in “Horizon cases” were founded upon POL’s failures of investigation and disclosure.”  Accordingly, Ground 1 succeeded.
It was the Post Office’s extensive knowledge of Horizon’s technical problems, and their deliberate failure to comply with their disclosure obligations, which underpinned the success of Ground 2. The Court ruled “that if the full picture had been disclosed, as it should have been, the public interest in prosecution would have been heavily outweighed by the need to maintain public confidence in the criminal justice system.” 
In sum, “the failures of investigation and disclosure were…so egregious as to make the prosecution of any of the “Horizon cases” an affront to the conscience of the court. By representing Horizon as reliable, and refusing to countenance any suggestion to the contrary, POL effectively sought to reverse the burden of proof…” 
Whilst not addressed directly by the Court, the ruling ought to shake any excessive confidence prosecutors, public or private, may have in the “objectivity” of technological evidence. As algorithms and machines are increasingly trusted to make decisions, prosecutors must be more attentive to the role such evidence plays in shaping the criminal liability of ordinary persons.
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