Between 1999 and 2015, more than 900 sub-postmasters were wrongly prosecuted after faults with Horizon made it look like money was missing from branch accounts.
The Horizon IT system has been at the heart of what’s been called the UK’s most widespread miscarriage of justice.
But Monday’s report, external from financial and risk advisory company Kroll has revealed fresh concerns over its predecessor Capture, which was developed in-house by the Post Office’s information technology team.
“We consider that, based on available evidence, there was a reasonable likelihood that Capture could have created shortfalls for sub-postmasters,” the report said.
Investigators said despite “various control functions being in place” at the Post Office, there was an “acknowledgement both in internal and external communications during the 1990s that Capture had bugs and errors that varied in severity”.
“The burden placed upon sub-postmasters to implement fixes, which varied in complexity, together with a reliance on communications from Post Office Limited to identify these bugs and fixes, meant there was a high capacity for errors to go unnoticed,” the report said.
Kroll did not comment on whether any convictions arising from sub-postmasters using Capture could be considered unsafe.
The company conducted 21 interviews with former sub-postmasters and/or their families as part of its investigation.
In a small number of cases, it was unclear if the sub-postmasters had used Capture software, Kroll said.
Out of the 21 sub-postmasters, Kroll said 12 had been suspended by the Post Office, though two occurred after 1999 when Horizon had replaced Capture.
“Of these 12, eight were prosecuted, two resigned, and two were terminated from their employment,” Kroll said.
“A further seven sub-postmasters were never suspended, but advised Kroll that they ended up selling their branch, in part or whole, due to losses they sustained during Capture use.”
