Almost seven in 10 Post Office sub-postmasters have experienced an “unexplained discrepancy” on the Horizon IT system since January 2020, a survey has revealed.

It suggests faults remain in newer versions of the accountancy system still used by the Post Office, which has been at the heart of one of the worst miscarriages of justice in British legal history.

A poll of sub-postmasters by YouGov found that almost all of those who had difficulties with Horizon reported shortfalls in their accounts and most resolved them by using their branch’s money.

An inquiry into the Post Office scandal resumed on Monday and will focus on how the company, which is wholly owned by the government, operates today.

Between 1999 and 2015, more than 900 sub-postmasters were wrongly prosecuted after faulty software called Horizon made it look like money was missing from Post Office branch accounts.

Despite this, a version of the Horizon IT system is still in use in UK Post Offices.

In January the BBC reported comments from a sub-postmaster who said the system was still causing shortfalls.

YouGov said it found that nearly half of the sub-postmasters it spoke to were “dissatisfied with how the Horizon IT System currently operates”, compared to 25% who were happy with it.

It added: “The vast majority – 92% – of sub-postmasters surveyed reported experiencing some form of issue with the Horizon IT system in the last 12 months.”

One sub-postmaster told YouGov: “In my opinion Horizon is still flawed I regularly have unexplained discrepancies often altering daily or manifesting at balance.”

The most common problems reported were screen freezes and loss of connection, while more than half said they had experienced “unexplained discrepancies”, and others mentioned seeing unexplained transactions.

YouGov was commissioned to carry out the research by chair of the inquiry Sir Wyn William. It invited 6,532 current sub-postmasters to take part in its survey between 18 July and 15 August – 1,015 people responded.

In response to the survey, the Post Office did not comment on specific findings, but said: “We are focused on supporting the Inquiry to reach its independent conclusions.

“Hearing directly from former and current postmasters is an important part of this work. We are determined to learn lessons from the past and improve the organisation.”

On Monday, the inquiry into the scandal heard from Saf Ismail, a sub-postmaster who was appointed to the Post Office board as a non-executive director in 2021 in a move by the company to attempt to improve relations with sub-postmasters.

In his evidence, Mr Ismail, who has stepped back from his role on the board, said sub-postmasters on the board were not welcome and he was told that Post Office chief executive Nick Read wanted to reverse the set-up because they were “too awkward and too challenging”.

Mr Ismail also said he raised concerns with the board over some Post Office employees being redeployed from investigations teams that had worked on Horizon cases to the remediation unit set up to award compensation to sub-postmasters.

“I think it’s an insult,” he said. “A postmaster who was potentially terminated, wrongly prosecuted, has then got an individual who potentially has done all that damage to them and their life then giving them the compensation – that just does not sit well with me.”

YouGov also held a second survey regarding compensation payouts relating to the Horizon Shortfall Scheme (HSS). It found nearly half of the 1,483 HSS applicants who responded were dissatisfied with the scheme.

Just under half the sub-postmasters surveyed said they found the scheme hard to understand, while some 78% reported they had not received contact from a compensation case assessor.

“Among those with an application outcome, there was generally high dissatisfaction with various elements of the offer,” YouGov said.

“There was a sense that the process was too long, with a lack of transparency.”

The HSS is for sub-postmasters who were not convicted, or part of the Group Litigation Order (GLO) court action, but who believe they experienced shortfalls because of Horizon. This group will be offered a fixed payment of £75,000.

It is one of three compensation schemes available to more than 4,000 people affected by the scandal. The other two are the GLO scheme for the 555 former sub-postmasters who won their group lawsuit, and the Overturned Convictions Scheme.

Former sub-postmaster and campaigner Sir Alan Bates has hit out at the length of time it is taking for victims of the Post Office Horizon IT scandal to receive financial redress, particularly in relation to the GLO scheme.

The government said all 2,417 “original” Horizon Shortfall Scheme applicants have received an offer of compensation. However, the government recognised that was not the case for later applicants, and said it was “working tirelessly to bring them full, fair and swift redress”.

Jerry Brown, a sub-postmaster who has been running his branch in Hadleigh, Suffolk, for more than 17 years, told the BBC while he was “hopeful” for change, he believes the attitude of the Post Office towards sub-postmasters remains the same.

“We are always at the bottom of the pile,” said Mr Brown, who is also on the executive committee of the Voice of the Postmaster group and took part in the YouGov survey.

He added that executives he has spoken to in the past did not “seem to understand or care how difficult it is” to run a branch.

He said it was “impossible” to make any money from running a Post Office at the moment, with three members of staff on-site all the time and other overheads like business rates and electricity bills going up.

Liam Byrne, the Labour MP who has been leading parliamentary hearings on the scandal, said that redress claims must be processed much more quickly.

Byrne also told the BBC that the inquiry must confront “big questions” about culture at the top of the Post Office and the treatment of sub-postmasters.

The Post Office has said that it has already made efforts to improve culture and its relationship with sub-postmasters, but it acknowledges there is more to be done.

“The main difference today is that we operate with the mindset that we are working in partnership with our postmasters when approaching any issues,” the company said.