Paula Vennells is set to be questioned under oath at the Horizon IT inquiry.

She is due to be quizzed on her role in the Horizon scandal after more than 700 subpostmasters were prosecuted by the Post Office and handed criminal convictions between 1999 and 2015.

Here is a timeline of key events from the scandal:

– 1999

The Horizon IT system starts to be rolled out in Post Office branches across the UK.

– 2000

Alan Bates reports issues with the Horizon IT system.

– 2003

Alan Bates has his contract terminated after refusing to accept liability for the shortfalls in the accounts at his Llandudno branch in North Wales.

– 2004

Lee Castleton, from Bridlington, East Yorkshire, was found to have a £25,000 shortfall at his branch. He was made bankrupt after he lost his legal battle with the Post Office.

– 2009

Computer Weekly magazine breaks the story of seven subpostmasters beginning their fight for justice.

The Justice for Subpostmasters Alliance is established.

– 2010

Pregnant subpostmaster Seema Misra, who ran a Post Office in West Byfleet, Surrey, is jailed after being accused of stealing £74,000.

– 2012

Paula Vennells becomes chief executive of the Post Office.

– 2015

Post Office boss Paula Vennells tells the business select committee that there is no evidence of miscarriages of justice.

Post Office halts prosecutions of subpostmasters.

– 2017

Legal action is launched against the Post Office by a group of 555 subpostmasters.

– 2019

A High Court judge rules that Horizon contained a number of “bugs, errors and defects” and there was a “material risk” that shortfalls in Post Office branch accounts were caused by the system.

The Post Office agreed to pay out £58 million to the 555 subpostmasters.

Post Office boss Paula Vennells is made a CBE.

– 2020

The Post Office does not oppose 44 appeals against convictions of subpostmasters.

– 2021

A statutory inquiry looking into the failings of the Horizon system and the wrongful convictions of subpostmasters begins.

The Court of appeal quashes 39 wrongful crown court convictions.

– 2023

The Government announces that every wrongly convicted subpostmaster will be offered £600,000 in compensation.

– 2024

ITV drama Mr Bates Vs The Post Office airs for the first time.

The Metropolitan Police say they are looking at “potential fraud offences arising out of these prosecutions”, for example “monies recovered from subpostmasters as a result of prosecutions or civil actions”.

Paula Vennells voluntarily hands back her CBE.