Gareth Jenkins has just taken his seat in the inquiry room and will be asked to take an oath.

The room is packed with people with a deep and personal interest in his evidence.

Sean Seddon
Reporting from the inquiry

I'm here at the Post Office Inquiry in Aldwych House, central London.

Today's evidence is among the most keenly anticipated so far in this years-long process, which was set up to answer a fundamental question: how was this scandal allowed to happen?

Today's witness Gareth Jenkins is already inside the building with his legal team.

Over the next four days, he will face what are the likely to be very tough questions from the inquiry's lawyer and we're expecting to hear an awful lot of technical discussions about the mechanics of the Horizon system.

I'll be at the back somewhere behind the army of lawyers and solicitors, who are a semi-permanent fixture in this building now, trying to make sense of what it all means.

Nalini Sivathasan
Reporter

In 2010 the Post Office was dealing with 12 cases where sub-postmasters were blaming Horizon for shortfalls in their accounts.

Seema Misra was a test case for the Post Office, a way of showing that there were no problems with the Horizon system. Gareth Jenkins’ evidence as an "expert witness" was key to achieving this.

However, his duty was to the courts, not to the Post Office. It's unclear whether he was ever told that, and documents obtained by the BBC show that a Post Office lawyer wanted Jenkins to rephrase some of his testimony in a way which strengthened his defence of Horizon.

He also failed to tell the courts about a bug and Fujitsu's ability to remotely access sub-postmaster accounts.

Had the court known that, it could have stopped Misra's prosecution - as well as others - in their tracks.

Seema Misra was convicted of theft and sent to prison while pregnant.

Image source, Alamy

Gareth Jenkins is an engineer who helped design the Horizon software at the heart of the Post Office scandal.

He graduated with a maths degree in 1973 and went straight to work for Fujitsu - or ICL as it was then known.

Jenkins went on to become one of the firm's select "distinguished engineers" and worked for it for decades, right through the period this scandal was unfolding.

Jenkins' involvement with the Post Office began in 1996 when he was asked to design features of the Horizon programme, so he had a deep expertise in the programme's inner workings. The Post Office frequently called on him to give evidence in legal cases where people it was prosecuting were arguing the crimes they were accused of were due to problems with Horizon.

From 2005, Jenkins was pivotal in helping the Post Office in court but did not flag up crucial evidence about known problems with the system

He is being investigated by the Metropolitan Police on suspicion of perjury and perverting the course of justice.

For those of you whose introduction to the Post Office scandal was the ITV drama, the name Gareth Jenkins might not ring a bell.

Unlike senior executives like Paula Vennells and Angela van den Bogerd, who were catapulted to household fame - or infamy - by their onscreen portrayals, Jenkins did not feature in that series.

As far as the inquiry is concerned, however, he is a star witness.

Four days have been set aside for lawyers to question Jenkins, longer than any other witness so far, and that should give an indication of how important they feel his evidence is.

As one of the so-called architects of the Horizon system, he knew the system inside out and was called on by the Post Office to give assurances about its performance in court.

His name has come up repeatedly during these proceedings and the inquiry clearly feels it's impossible to get the full story without hearing from Jenkins at length.

Emma Simpson
Business correspondent

We've heard so much about Gareth Jenkins, especially during this phase of the inquiry. His name is popping up almost every day.

Finally, it's now time to hear from the so-called architect of the flawed Horizon IT system. It's fair to say his appearance will be keenly anticipated.

From 2005, he was pivotal in helping the Post Office defend Horizon in criminal and civil cases. He was used as an expert witness testifying that the system was robust but he failed to tell the courts about computer bugs. This was crucial evidence which should've been disclosed.

The Post Office was warned it was in breach of its duty as a prosecutor his credibility was "fatally undermined".

The inquiry's heard he was never properly instructed.

Jenkins is investigated by the Metropolitan Police for potential perjury. A lawyer representing him said it would be "inappropriate" for him to comment ahead of him giving evidence at the inquiry.

Emily McGarvey
Live editor

Welcome to our live coverage of the Post Office inquiry. Gareth Jenkins, the former Fujitsu engineer who helped design the faulty Horizon software at the centre of the scandal, will begin giving evidence today.

Jenkins will be questioned over his involvement in the Post Office scandal, which saw more than 900 sub-postmasters wrongly prosecuted between 1999 and 2015 after Horizon made it look like money was missing from branch accounts.

He will give evidence over four days - the longest run of questions any witness has faced.

You can watch the inquiry live from 09:45 BST by clicking play at the top of this page and we'll have live text updates here.