Post Office IT support email reveals known Horizon flaw

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An email from Post Office IT support reveals a problem with the Horizon system and supporting processes that could lead to accounting errors

Technicians supporting the Post Office's Horizon IT system have revealed a known software flaw that could lead to subpostmasters submitting inaccurate accounts.

Post Office IT support sent an email to a member of the postmasters' branch of the Communications Workers Union (CWU) revealing the flaw. The email said Post Office IT support planned to fix the glitch “with a code change” in early 2016. Subpostmasters have not been warned about this flaw in the system.


Campaigners blame the Horizon computer system (see panel below) for accounting shortfalls that have led to subpostmasters being fined and even jailed. The Post Office has consistently denied this.

In November 2015 the CWU wrote to subpostmasters warning them of a problem with the system, following an incident in which thousands of pounds' worth of payments were duplicated for one subpostmaster. If undetected, this would appear as losses when the accounts are completed, which would be the responsibility of the subpostmaster.

The Post Office told Computer Weekly it had given the affected subpostmaster a “full outline” of the cause of the duplication of payments, but would not give details.

Previous instances

But in an email to another postmaster, who is also a CWU member, about the case of duplicated payments, Atos – the company that provides IT support for the Post Office – described the problem, and said it had happened before.

In the email – which Computer Weekly has seen – an Atos representative said: “This issue is caused by the user forcing log off when the post-login checks have not fully completed. We have experienced previous instances of this issue in other branches [that] have been caused in the same way (forced log off).”

The Atos employee said the problem is a process issue that would require a code change, from Horizon supplier Fujitsu, to stop it happening again across the network. The email said the code change was planned for March 2016.

“The ability for this to occur can be addressed by a code change that will avoid further instances of this across the estate,” said the email. “The code change will be included within Release 13.05 which will be developed by Fujitsu and deployed in March 2016."

What is Horizon?

In response to a Freedom of Information (FOI) request made in 2011 by the Justice for Subpostmasters Alliance (JFSA) campaign group, the Post Office explained the scope of the term "Horizon".

“This encompasses the software, both bespoke and software packages, the computer hardware and communications equipment installed in branch and the central data centres. It includes the software used to control and monitor the systems. In addition I can advise you that testing and training systems are also referred to as Horizon,” said the Post Office.

The Atos helpdesk has closed the case, but the CWU postmasters branch claimed the supplier had not investigated it properly.

Mark Baker, postmaster branch secretary at the CWU, said the explanation given by Post Office IT support could not be the right one regarding duplicated payments, and claimed it was evidence of a different flaw.

“This subpostmaster only had one terminal and forced log-off only occurs when you try to use a terminal when already logged in to another,” he said. He added that it could not have been caused by the system logging off automatically after being unused for a while, because records show the transaction was made after the previous log-off happened.

The Atos email said: “From an IT perspective we have agreed that this can be closed as the incident is resolved. That said, we are aware of the ongoing work the Finance Service Centre (FSC) will need to do with [the subpostmaster] and we have asked this team to pick it up and I understand contact has been made.” The subpostmaster did talk to the FSC to have the duplicated transactions corrected, but has had no contact since then, said the CWU.

The Horizon dispute

The possibility of a flaw in Horizon that could cause undetected accounting errors is critical in light of the disputes between some subpostmasters and the Post Office that have been running for years. In 2009 Computer Weekly revealed the stories of subpostmasters who had received heavy fines and even jail terms for alleged false accounting – which they blamed on the system and supporting processes. The Post Office has vehemently denied this claim. (See timeline below for more about the dispute).