The former head of IT at the Post Office named by the ex-chief executive Paula Vennells as one of five people responsible for the Horizon scandal has said he still has not seen evidence of a "fundamental flaw" in the IT system.

Mike Young, who worked at the Post Office from 2008-2012 when the legacy Horizon was moved to an online system, was giving evidence to the public inquiry into the scandal on Tuesday, months after it was told "he could not be found" to add him to the witness list.

He told the hearing: "I did not see anything fundamentally wrong with Horizon, or its IT code, at the time, other than minor bugs and glitches that would normally be expected in a pilot.

"Still, to this day, I am unaware of an identified part of Horizon code that someone can point to show that Horizon is fundamentally flawed. I understand Horizon Online is still used today."

Young, who served in the army and rose to the rank of detective sergeant in the police before entering the business world, also defended the ability to remotely access IT systems.

The fact Horizon's developer Fujitsu was able to remotely access and change branch accounts, which the Post Office long maintained was not possible, was a key plank of wrongly prosecuted post office operators' legal argument for justice.

"I was unaware of any remote or privileged access being exercised without the consent of the postmasters or sub-postmasters," said Young.

"To be clear, most software in the world can be accessed remotely, as this is a necessary operation for any modern IT support function. For example, the National Security Agency, Central Intelligence Agency ... can all be accessed remotely. Remote access to IT systems is the standard used by IT support functions to try and resolve issues quickly and efficiently."

The inquiry saw evidence that despite Young being comfortable with the integrity of Horizon, he did try to get Fujitsu's agreement to be "open book" and call in an independent third-party review of the code at the heart of the IT system.

Fujitsu ultimately refused to cooperate with the request, with the Post Office unable to go further under the terms of its contract with the Japanese software company.

Young said Fujitsu had argued: "It is our intellectual property and we don't want to share it. We own the intellectual property and you have no right to see it." He told the inquiry: "I don't agree with that. I have never agreed with that."

"To this day, I do not believe a full investigation into the Horizon code has taken place to determine whether there was an actual code issue with Horizon or just the processes followed and training around it being deficient," said Young. "In my opinion, the Post Office needed one of the big forensic consultancy firms to look at the entire Horizon operating framework, inclusive of process, training and, most importantly, the Horizon code itself."

Earlier this year, the inquiry heard from a Fujitsu software developer who raised the issue of bugs in the Horizon IT system but said the company did not properly fix the problem because it would have been too expensive and time-consuming.