It will be the second time that Paul Patterson, the director of Fujitsu Services Ltd, faces a grilling on the scandal after appearing at the Business and Trade Committee on Tuesday.
Mr Patterson previously said the technology giant has a “moral obligation” to contribute to compensation for subpostmasters.
He also apologised to subpostmasters who had been wrongly convicted as a result of problems within the Horizon software and said there were “bugs and errors in the system”.
In phase four of the public inquiry today, questions will look at the action taken against subpostmasters – including civil and criminal proceedings and failures in investigations.
Mr Patterson’s evidence today comes after several Fujitsu witnesses – including security team managers and software developers – were quizzed on their roles when dealing with problems reported by subpostmasters, and subsequent prosecutions.
One Fujitsu manager, Peter Sewell, was grilled about a 2006 email in which he called subpostmaster Lee Castleton a “nasty chap” who will be “all out to rubbish” the company’s name ahead of legal proceedings which led to his bankruptcy.