It has been described as the worst miscarriage of justice in recent British legal history.
More than 700 Post Office workers were wrongly prosecuted for stealing money from the company, after a computer system called Horizon incorrectly showed shortfalls on their accounts.
Most were asked to make up the balance out of their own pocket when they flagged the issues, and many were prosecuted, ending up in prison, or bankrupt.
Among them was former subpostmaster Parmod Kalia, 63, whose conviction for theft was overturned last year.
The conviction had a devastating impact on his personal life. He attempted suicide three times in 2015, and his own family doubted his innocence for years.
“It felt very shameful,” he said.
“My job was to care for the family, look after them. And I couldn’t do it.”
The father-of-four was speaking to an inquiry into the failings of the Horizon IT computer system, which was developed by Fujitsu.
The inquiry aims to establish a clear account of the system’s faults, and whether the Post Office knew about them.
Just weeks after the system was introduced in Mr Kalia’s Orpington post office in 2000, regular shortfalls started showing, he said.
“I felt very disturbed,” Mr Kalia told the inquiry.
“I would quite often be in the post office balancing until midnight or 1am with my wife, with my little girl – she was six or seven at the time, she was sleeping there.
“She’d have to come out early in the morning with us, and she would have to sleep behind the counter until we finished – it could be 12 o’clock or one o’clock in the night.”
An audit found a £27,000 shortfall.
‘The plans that we’ve made for the children’s education had all failed’
Mr Kalia said that, on the advice of a representative of the National Federation of SubPostmasters, he made up the loss by asking his mother for her life savings, which caused resentment among his siblings.
Nonetheless, he was prosecuted for theft, and advised by the same federation representative to plead guilty, he told the inquiry, in return for a more lenient sentence.
It led to a six-month custodial sentence, three of which he served at home with an ankle tag, and a sharp decline in his family’s fortunes.
“It felt as though I was letting the family down again,” Mr Kalia told the inquiry. “I couldn’t support, care for my family as I should have been. The plans that we’ve made for the children’s education had all failed.
“My second son, he was in an independent fee-paying school before the audit and he was finishing school to go to university. And my oldest daughter was just turning 11.
“Before Horizon I had already had her admitted to go the same fee-paying school. I had to take her out of there, had to remove her, and she had to be admitted to an ordinary state school.”
‘I disrupted their education’
“They were bright, capable children, but I disrupted their education. She (had) made friends there and she told me that she lost her friends there for good,” an emotional Mr Kalia told the inquiry.
He was eventually awarded compensation of £27,000, but even though the government agreed to pay £100,000 to the wrongly-convicted postmasters, Mr Kalia said that he was denied that payment.
“Why has this payment been denied? Is it because of colour?” Mr Kalia, who is British-Asian, asked the inquiry.
Another witness Chris Trousdale, who did get the payment, said it was hard to quantify the financial losses he suffered over 20 years.
His family ran post offices for 150 years, but he was just 22 when he was accused of a shortfall and sentenced to a community punishment order in March 2004, after being prosecuted by the Post Office and advised to plead guilty to false accounting or face jail.
Suspicion never went away
Even though his conviction was quashed last year, he told Sky News that in the wider community, suspicion never went away.
“You start to get this guilt that you dragged people into this situation – even though you’re not responsible for the situation, you have this massive guilt that you’ve dragged your family and friends into it, and that’s difficult to get over,” he said.
Mr Trousdale said that he was told by the Post Office that the Horizon system was “100% robust” and that no one else was having issues with it – something that was echoed by other witnesses at the inquiry.
‘They isolated us all’
“They isolated us all – there would be power in numbers you see, they kept everyone apart,” he said.
“We were told: ‘you’re the only one having a problem with Horizon system, you’re the only one that’s having these mistakes’, and as we know now that was the modus operandi they rolled out – stop people coming together and fighting this, which we now have.”
The inquiry is expected to deliver its findings next year.