A shortfall was the beginning of the inquiry
A post office branch ends a period with less money than its accounting system says should be present. One explanation is theft. Others include a recording problem, an error in reconciliation or a fault in the system. A prosecution relying on that discrepancy needs a way to establish a real loss and connect it to the person charged.
The Post Office’s Horizon cases exposed what can happen when those steps are collapsed into one. On April 23, 2021, the Court of Appeal quashed 39 convictions referred by the Criminal Cases Review Commission. Its judgment in Hamilton and others considered how the Post Office investigated apparent shortfalls and what it disclosed about the system to people facing criminal charges.
Why disclosure changed the outcome
When Horizon figures were central to the case and there was no independent proof of an actual loss, the defense needed enough information about the system to challenge the figures. The court examined failures to investigate those problems and share material that could have helped the defense. Without that opportunity, a defendant might face a figure they were expected to explain while having no practical way to test how it was produced.
The court found that in the Horizon-dependent cases it examined, the failures made a fair trial impossible. Its judgment also addressed whether bringing those prosecutions amounted to an abuse of the court’s process. The distinction matters because the decision concerned both the fairness of individual proceedings and the way the prosecutions had been brought.
Why a guilty plea could still be unsafe
One appellant, Pauline Thomson, had pleaded guilty to false accounting. The judgment notes that she acknowledged an apparent shortage at her branch and that the defense had raised the possibility of Horizon error. The court found the figures essential to the prosecution and found no proof of an actual loss separate from the system’s reported shortfall. It quashed her convictions.
A plea closes many avenues of dispute at trial. The appeal court still had to ask how the plea came about and whether the earlier failures had undermined the process that led to it. The guilty pleas of successful appellants did not stop the court from examining that issue.
Why the court assessed each case separately
The Criminal Cases Review Commission records that three other referred appeals were dismissed. The court concluded that Horizon evidence had not been essential to those prosecutions. Its treatment of those cases helps explain the reach of the ruling. What mattered was the role the disputed system played in proving the specific charge.
The Hamilton judgment covers a defined group of appeals decided on a particular day. It remains useful because the court sets out how to examine computer-generated evidence in context: identify what the number actually measures, look for independent corroboration and ask whether the defense had enough information to test the claim.