Ms Vennells was chief executive officer of Post Office Ltd between 2012 and 2019.
Former Post Office boss Paula Vennells will give evidence at the next stage of the inquiry into the Horizon IT scandal.
Ms Vennells, who was chief executive officer of Post Office Ltd between 2012 and 2019, has come under fire over why hundreds of subpostmasters were wrongly prosecuted for fraud and false accounting under her watch.
Glitches in the Horizon IT system used by the Post Office meant money looked as if it was missing from many branch accounts when in fact it was not.
The scandal, which was ongoing from 1999 until 2015, represents one of the biggest miscarriages of justice in UK legal history and more than 100 subpostmasters have had their convictions quashed by the Court of Appeal.
Many more are yet to be cleared and the Government has come under fire for the compensation awarded to victims.
Ms Vennells, who joined the Post Office in 2007, handed back her CBE for services to the Post Office and to charity last month after public anger following the screening of ITV drama Mr Bates vs The Post Office.
After handing it back she told the PA news agency in a statement that she will “support and focus on co-operating with the inquiry”.
She added: “I have so far maintained my silence as I considered it inappropriate to comment publicly while the inquiry remains ongoing and before I have provided my oral evidence.
“I now intend to continue to focus on assisting the inquiry and will not make any further public comment until it has concluded.”
Alan Bates will also give evidence to the inquiry when it resumes in April.
Others set to testify in phases five and six include Lord Arbuthnot of Edrom, who campaigned on behalf of subpostmasters while an MP, former business secretary Sir Vince Cable and Liberal Democrat leader Sir Ed Davey, who has been criticised over what more he could have done while postal affairs minister during part of the scandal.
