The wrongful convictions of hundreds of sub-postmasters and mistresses will be examined by a public inquiry starting on Monday.
Between 2000 and 2014, more than 700 sub-postmasters were wrongly accused of theft, fraud and false accounting due to a flaw in a computer system Horizon.
A total of 72 former sub-postmasters have had their names cleared so far.
The cases constitute the most widespread miscarriage of justice in British legal history
The inquiry – which is expected to run for the rest of this year – will look at whether the Post Office knew about faults in the IT system and will also ask how staff shouldered the blame.
It will also examine whether staff at software firm Fujitsu, which developed the Horizon software to complete tasks such as transactions, accounting and stocktaking, knew the system had flaws while data from it was used in court to convict sub-postmasters.
A judge will hear evidence on why sub-postmasters and postmistresses were singled out and whether they have been justly compensated.
Ahead of the inquiry, Baljit Sethi, who will be the first witness to give evidence on Monday, told the BBC he was looking forward to it.
“What the Post Office has done to us – and to many of my colleagues – is unforgiveable,” he said. “The people who did this should be brought to justice.”
His wife Anjana, who also helped run two branches near Romford in Essex, said she now felt “we can see some light at the end of this dark tunnel”.
One branch had no problems at all, but the second one showed a hole in the accounts of £17,000, which they were asked to cover out of their own pocket. The Sethis were never charged but still faced a financial and emotional nightmare.
Their son Amit added: “I want accountability and those who suffered should be compensated appropriately.
“That’s it – then we’ll draw a line under it.”
Harjinder Butoy, who ran the post office in the Nottinghamshire market town of Sutton-in-Ashfield along with his wife, said that he wanted to see “someone on the other side to be charged and jailed like I was”.
“They are the ones who signed the final paper work off,” he told the BBC. “They are the ones who knew if there were any faults on the system or not.”
Mr Butoy, who is thought to have been given the longest prison sentence at three years and four months, was wrongly convicted and jailed for stealing £208,000 and it took until last year to get his conviction overturned in the High Court.
Mr Butoy said he noticed the Horizon accounting software was not adding up properly.
“I was £500 short,” he explained, even though a supervisor had been with him the entire time, making sure he got to grips in his new role. “I didn’t think anything of it. But ever since then, it happened a lot.”
Thousands of others Post Office sub-postmasters lost huge sums of money as they tried to make up mysterious shortfalls at their shops, which the Post Office is only now beginning to refund.
Many have described being shunned by their communities while some have since died.
The Post Office has said it is “sincerely sorry” for the impact of the Horizon scandal, adding it is “in no doubt about the human cost”.
It said the inquiry would enable “many of those who were most deeply affected by Post Office’s past failings to voice their experiences and their testimonies must and will ensure all lessons are learned so that such events can never happen again”.
“In addressing the past, our first priority is that full, fair and final compensation is provided and we are making good progress,” it added.
The government has set aside funding to facilitate compensation payments.
Meanwhile, Fujitsu said it was “committed to providing the fullest and most transparent information so that key lessons are learned for the future”.