A former sub-postmaster who claims he was wrongly jailed for accounting discrepancies has said he feels bitter.
Noel Thomas was jailed for nine months in 2006 after he admitted £48,000 in false accounts while he was employed by the Post Office in Gaerwen on Anglesey.
On Wednesday, he will join 556 former colleagues, some of whom claim their prosecution was due to problems with the Horizon computer system.
The Post Office said it will "robustly defend" itself against the claims.
"I want everyone to have their name cleared and to get to the bottom of what has happened and where the money has gone to," Mr Thomas told BBC Newyddion 9.
Mr Thomas said he admitted to the charge because he never reported discrepancies he noticed, but insisted he did not take the money and blames the computer system.
A BBC investigation in 2015 found that "operational errors" could have led to false accounting.
"Thirteen years after jail, I must admit it was hard but I gradually got my confidence back through family, friends and work colleagues," he added.
"Yes, I do feel bitter, and not just for myself - the Post Office have been coming and telling people that they have taken money, that they are a thief."
He added: "I am certain the computer systems are at fault."
At the High Court in London on Wednesday, the Post Office will deny his claims and those of hundreds of his former colleagues.
The class action was started by Alan Bates who was a sub-postmaster from Llandudno Junction between 1998 and 2003.
A Post Office spokesperson said: "The Post Office will be robustly defending its position in the court and welcomes the opportunity to do so.
"We take these cases extremely seriously and we have worked hard over a number of years to address the issues raised.
"We have conducted thorough investigations and sought to resolve some of the claims through mediation.
"It's important to note that the claimants represent a very small proportion (0.01%) of users of our Horizon computer system users since 2000."
This 20-day trial will deal with their contracts. A second trial in March will examine the IT system and further trials next summer will look at individual cases.