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Post Office six years late to warn subpostmasters about Horizon defect

Following pressure from Computer Weekly and forensic investigator, the Post Office has warned subpostmasters about Horizon defect potentially at large for over 20 years

Karl Flinders
By
Published: 18 Dec 2025 16:15

The Post Office has issued information and instructions to its branch network regarding an issue with Horizon, six years after it began “fobbing off” a subpostmaster who raised concerns about it on numerous occasions.

After pressure from Computer Weekly and forensic investigator Ron Warmington, the Post Office finally investigated the issue, first raised in June 2019, about a defect in Horizon that could create unexplained account shortfalls in branches.

The issue, branded the “part cash” defect by Denis O’Donnell, the former subpostmaster who raised concerns with the Post Office, was likely to have seen some customers, for decades, receive extra cash from subpostmasters without realising. A Horizon screen icon which has two opposite uses, allowing the subpostmaster to both take money and pay money, is at the centre of the issue.

The Post Office, under new leadership, agreed to look into the Horizon defect earlier this month, but O’Donnell said he was “fobbed off” by the Post Office for many years over the issue. It was 2019 when he first alerted the Post Office with letters to senior executives including former CEO Nick Read and Mark Davies, who headed up the Post Office’s communications department.

O’Donnell’s first letter stated that it was “regarding a possible systemic problem”. This was at a time when the Post Office system was being interrogated in a High Court trial that ended with it admitting Horizon errors could cause the shortfalls it had blamed and punished subpostmasters for, something it had previously denied.

Following a second letter a month later, Davies replied, writing that the Post Office was looking into the problem. O’Donnell continued to write to the Post Office and was repeatedly told they were looking into it, but it was never resolved.

Computer Weekly revealed the issue to Warmington at forensic investigation firm Second Sight, which helped to expose the Post Office Horizon scandal. He said he found it serious enough to write to the Post Office scandal public inquiry and report what he had learned.

Meeting with Post Office IT boss

Following a Computer Weekly article regarding the defect, things escalated quickly, with O’Donnell and Warmington meeting with Post Office IT boss Paul Anastassi and other executives to discuss the issue.

According to Warmington, during the meeting, it emerged that account shortfalls being experienced today in branches could be caused by the issue. He said it is only a small number today, but is likely to have been much larger in the past.

A Post Office spokesperson said: “We recognise that the issue Mr O’Donnell describes does exist and does create a discrepancy. It is not due to a technical defect with Horizon, but an issue with process (user journey). However, we do recognise that the possibility for that incorrect user journey does exist on the system still and, if used, results in a discrepancy.”

Read more about the ‘part cash defect’

Warmington demanded that the Post Office fix the defect and “pending that”, warn subpostmasters and their staff about it, so that they can reduce the risk of future losses.

He also called for the Post Office to “admit that, for over 25 years, this defect will have been generating shortfalls in branches”. Although, he said: “It’s probably impossible to determine how often the problem has caused branch losses, nor to whom.” 

The Post Office spokesperson said: “It is impossible to know the exact impact in the network and we acknowledge there will be postmasters who have experienced the issue and not raised it. But we do believe the overall impact is limited.”

It said from going through its records, the number of discrepancies in a year attributed to this process is 10 out of 14,000, or 0.07%, where discrepancies have been made known to it.

“We would like to assure your readers that Post Office today has taken this matter seriously and our chief technology officer, chief corporate affairs officer and central operations director met virtually with Mr Warmington and Mr O’Donnell twice,” said the Post Office. “We encourage any current or former postmaster who believes they may have been impacted by this issue to get in touch as they may be eligible for redress through the Post Office Process Review.”

O’Donnell praised the new Post Office leadership for their actions to address the issue and supported the demands made by Warmington. “We’ve got to respect the fact that the [new leadership] has changed tack on this,” he said. “I think it’s fantastic what’s occurred in the last few months. I think that it is a marvellous change for me because I’ve been carrying this alone for a long time.”

The Post Office scandal was first exposed by Computer Weekly in 2009, revealing the stories of seven subpostmasters and the problems they suffered due to the accounting software (see timeline of Computer Weekly articles about the scandal below).

Timeline: Computer Weekly articles about the scandal since 2009

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