At least 18 people have now come forward raising concerns over a second IT system used by the Post Office, Sky News understands.

Accounting software, Capture, was installed by several post offices around the country in the mid to late 90s – before the notorious Horizon system was rolled out.

Hundreds of sub-postmasters were wrongly convicted after errors in the Horizon software caused false accounting shortfalls.

Lawyers for the victims of the Post Office scandal have told Sky News more people who used the Capture system may now come forward.

Sources have also said there are growing concerns about a continued “culture of denial” at the Post Office, and a suspicion that record-keeping was “in a mess”.

Documents show that Capture was known by the Post Office to have issues early on.

A Post Office spokesperson said they take any concerns “very seriously”.

They are also “particularly concerned about allegations of prosecutions”.

Their statement continues, saying that they are looking into “whether shortfalls could have been caused by faults in this software, and the potential impacts if so”.

Given the passage of time, and changes to data storage they say they “do not yet have a complete picture”.

The Capture software system was not believed to have been “networked” to a larger, wider, system but was used by some postmasters across the country.

Former sub-postmasters have told Sky News they had to pay more than £1,000 for the software despite it being developed in-house by the Post Office.

Steve Marston, now 68-years-old, pleaded guilty to theft after shortfalls of over £79,000 showed up in the year between 1996 and 1997 when he started to use the Capture software.

For the twenty years previous, he said he had had “no issues” with accounting.

Mr Marston had received bravery awards from the Post Office in the past for confronting armed robbers on two occasions.

He describes feeling “betrayed” by the Post Office who, he says, missed an opportunity ahead of the Horizon scandal.

“I mistakenly thought Capture was a computerised system and computers don’t make mistakes.

“I didn’t know any better. I just automatically assumed it was something I was doing wrong.”

He was spared prison but spent 12 months on probation and still feels the “shame”, saying it has ruined his and his family’s life.

After his conviction, he sold his wedding rings and wife’s jewellery to “make ends meet”, and spent years living in a caravan because he lost his house.

“I just felt so guilty – I still feel shame and guilt about it.”

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Rupert Lloyd Thomas, a former IT specialist for the Post Office for 27 years, said he had repeatedly raised concerns about how the Post Office managed its software systems.

He was a local manager in Birmingham in the mid-80s, at one point, and says he often went into offices to see what was happening.

“I remember visiting an office in west London, in Hounslow,” he said, “and I opened a cupboard there and there were hundreds of floppy disks sitting in this cupboard sort of discarded and I said well, what’s all this?

“This was software crashing in the office, and yet I get back to headquarters in London. Nobody’s interested. You know, we’re getting a new system. So why would we worry about mending the old one.

“All of those things went on. I think a lot of these people that have appeared in front of the Post Office inquiry genuinely didn’t know what was happening because they didn’t bother to go find out.”

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Kevan Jones, MP, who has supported Horizon victims, says he is in touch with around ten possible victims of Capture.

“What makes me very angry,” he says, “is the fact that the Post Office, despite all the publicity around Horizon, didn’t feel necessary to come forward and admit that they had a system before Horizon where similar things occurred.

“They haven’t raised that at the public inquiry, they haven’t raised it with ministers.

“And I think certainly Nick Reed the chief executive needs to come clean now and ask why they’ve not done that.”

The Department for Business and Trade has said that it “is in active discussions with the Post Office about the Capture system issue and are taking it very seriously”.

“If there is evidence that this system led to improper accusations, the government will not hesitate in taking robust action.”

It is also understood that there is a possibility the public inquiry could be expanded to include the Capture system if more evidence emerges.