Vennells was the CEO of Post Office Ltd during the latter part of the Post Office scandal, which involved more than 900 [[Sub-postmaster|subpostmasters]] being wrongly convicted of [[theft]], [[false accounting]] and [[fraud]] between 1999 and 2015 because of shortfalls at their branches that were in fact errors of the Horizon accounting software used by the Post Office.<ref>{{cite web |last=Seddon |first=Sean |date=2024-01-11 |title= How do the Post Office scandal compensation schemes work? |url=https://www.bbc.com/news/uk-67942935 |website=BBC News |publisher=BBC |access-date=2024-01-29 |quote=...There have been 983 convictions - 700 of which were privately initiated by the Post Office - linked to the faulty Horizon IT programme}}</ref> In addition over 2,000 subpostmasters paid for shortfalls caused by Horizon or, as forand many, had their contracts terminated. The actions of the Post Office caused the loss of jobs, bankruptcy, family breakdown, criminal convictions, prison sentences and at least four suicides.<ref name="Guardianexplain2">{{cite web |last=Sweney |first=Mark |date=7 January 2024 |title=What is the Post Office Horizon IT scandal all about? |url=https://www.theguardian.com/business/2024/jan/07/what-is-the-post-office-horizon-it-scandal-all-about |work=[[The Guardian]] |access-date=8 January 2024 |archive-date=30 May 2024 |archive-url=https://web.archive.org/web/20240530052700/https://www.theguardian.com/business/2024/jan/07/what-is-the-post-office-horizon-it-scandal-all-about |url-status=live }}</ref><ref>{{cite web |date=10 October 2022 |title=Post Office and Fujitsu to face inquiry over Horizon scandal |url=https://www.bbc.co.uk/news/business-63205335|work=BBC}}</ref> In 2013, Post Office Limited hired forensic accounting firm Second Sight, headed by Ron Warmington, to investigate the Horizon software losses. Warmington discovered the system was flawed and faulty, but Vennells was unhappy with Warmington's report and terminated their contract.<ref>{{Cite web |title=Second Sight's Ron Warmington breaks his silence |url=https://www.postofficetrial.com/2019/12/second-sights-ron-warmington-breaks-his.html |access-date=10 January 2024}}</ref> Prior to her role as CEO, Vennells was the Chief Operating Officer of Post Office Ltd, a position in which{{snd}}according to the evidence of the then CEO, David Smith{{snd}}she had responsibility for management of the "operational use" of the Horizon software.<ref name="WITN05460100">{{cite web |title=WITN05460100 David J Smith - Witness Statement |url=https://www.postofficehorizoninquiry.org.uk/evidence/witn05460100-david-j-smith-witness-statement |website=Post Office Horizon IT Inquiry |access-date=14 April 2024 |date=11 April 2024}}</ref>{{rp|12}}
Acting as a [[private prosecutor]], the Post Office repeatedly failed to make full disclosure of known Horizon problems either to defendants or to the courts in hundreds of cases. According to the [[Criminal Cases Review Commission]] the nondisclosure is "the most widespread miscarriage of justice the CCRC has ever seen and represents the biggest single series of wrongful convictions in British legal history".<ref>{{cite web |author=<!--Not stated--> |date=3 January 2024 |title=The CCRC and Post Office/ Horizon cases |url=https://ccrc.gov.uk/news/the-ccrc-and-post-office-horizon-cases/ |access-date=7 January 2024 |website=ccrc.gov.uk |publisher=Criminal Cases Review Commission |quote=}}</ref>
