A confidential report has criticised the Post Office over its handling of computer problems which led to some sub-postmasters being accused of theft.
More than 150 sub-postmasters claim they were wrongly prosecuted, or made to repay money, because of the system.
The report - which the Post Office commissioned and has been seen by BBC News - said the technology was not fit for purpose in some branches.
But the Post Office said there was "no evidence" of systemic computer issues.
Sub-postmasters run smaller post offices, and are not directly employed by the Post Office.
Under their contracts, they are responsible for deficits at their branches.
But after the Horizon transactions processing system was introduced in 1995, some sub-postmasters found that money from bank machines, lottery terminals, and tax disc sales did not match their computer's figures.
The report said investigators did not look for the root cause of the errors - and instead accused the sub-postmasters of theft or false accounting.
Last year, Jo Hamilton - who used to run a sub-post office from her village shop in South Warnborough, Hampshire - told the BBC that the system had made up a £36,000 deficit in her branch.
Another sub post-mistress, Sarah Burgess Boyd from Newcastle-upon-Tyne, said she lost her life savings in repaying an incorrect shortfall.
The report, which was carried out by investigators Second Sight, said training was not good enough for those without IT skills.
It also claimed that equipment was outdated, and that power cuts and communication problems made things worse.
An interim report was published last year.
The vast majority of the 11,500 sub post offices have used the system without a problem, but the Post Office is now in mediation with those who say they were treated unfairly.
In a statement, the Post Office said: "Although we will not comment on the contents of any confidential documents, after two years of investigation it remains the case that there is absolutely no evidence of any systemic issues with the computer system which is used by over 78,000 people across our 11,500 branches and which successfully processes over six million transactions every day."