Former sub-postmaster Lee Castleton is suing the Post Office and Fujitsu for more than £4m in damages over the Horizon IT scandal, court documents reveal.

Mr Castleton is one of the most high-profile of hundreds of sub-postmasters who were wrongly convicted after faulty software said money was missing from their branch accounts.

He became the first individual to take legal action against both organisations and this is the first time full details of a complex compensation claim have been made public.

The Post Office said it could not comment on ongoing legal proceedings but was “engaging fully” in the process.

Mr Castleton was portrayed by actor Will Mellor in the hit ITV drama Mr Bates vs the Post Office. The former sub-postmaster was awarded an OBE for services to justice in recognition of his tireless campaigning.

Speaking to the BBC about his £4.487m claim he said: “I want it to be made public. This is what they did to me and my family.

“It’s not about the money. What matters to me is that I get vindication from the court.”

In 2007, Mr Castleton lost a two-year legal battle against the Post Office after it pursued him to recover £25,000 of cash it alleged was missing from his branch in Bridlington, East Yorkshire.

When his legal insurance ran out, Mr Castleton represented himself in court and was landed with a bill of £321,000 in legal costs which he couldn’t pay and declared bankruptcy.

His was the only civil claim the Post Office brought against a sub-postmaster.

The official inquiry into the scandal heard evidence that the Post Office knew Mr Castleton would likely be made bankrupt by the action but wanted to make an example of him to dissuade others from pursuing claims.

The court documents reveal that in Mr Castleton’s case his quantifiable financial losses include:

• £940,000 past lost earnings plus interest
• £864,000 future loss of earnings
• £933,000 past pension losses
• £133,000 past property losses
• £232,000 past losses of rental profits plus interest
• £109,000 loss from sale of business plus interest

He’s also seeking general damages – these are losses that can’t be measured in pounds and pence. They include:

• £30,000 for mental distress plus interest
• £30,000 for stigma and damage to reputation plus interest
• £45,000 for harassment
• £50,000 for maliciously causing his bankruptcy

“When your life, as well as your family’s, has literally been ruined it results in a substantial claim,” said his solicitor Simon Goldberg, from Simons Muirhead Burton.

“The reason it’s so startling is that it’s the first time that the forensic details of a sub-postmaster’s claim been made public. Like many others, Lee has a very complex case, and the figures have been calculated by experts who are leaders in their field,” he said.

Mr Castleton has never applied to the relevant compensation scheme after losing faith in the fairness of the process. He wants a judge to decide what he is owed and to have “justice” through the courts.

His legal team allege that the Post Office’s decision to pursue a civil claim against him was an “abuse of process of the court.” And that the eventual judgment against him was obtained by fraud.

They also all claim the state-run institution conspired with Fujitsu to pervert the course of justice by “deliberately and dishonestly” withholding evidence.

This included knowledge of bugs and errors as well as the issue of remote access – the ability of some Fujitsu employees to access sub-postmasters’ branch accounts without their knowledge.

The Japanese owned company developed the software and is responsible for operating and maintaining the Horizon IT system.

Mr Castleton was one of the 555 sub-postmasters who took part in the landmark court case against the Post Office and won.

Both sides agreed to end the legal dispute. But Mr Castleton claims the settlement doesn’t apply to his current claims as well as alleging it was obtained by fraud.

Specifically, he argues the Post Office concealed the true reason why the former Fujitsu software engineer, Gareth Jenkins, wasn’t called as a witness at the trial.

Mr Jenkins provided testimony in a number of prosecutions. But in 2013, the Post Office was warned that he had failed to disclose information “in plain breach of his duty as an expert witness”.

The sub-postmasters weren’t told about the concerns as they fought their case.

Mr Castleton is seeking both the civil judgement and the bankruptcy order against him to be set aside on these grounds.

A Post Office spokesperson said: “We recognise the devastating impact of the Horizon IT Scandal on former postmasters like Mr Castleton. Post Office today is committed to doing all we can to help those affected get closure.

“We cannot comment on ongoing legal proceedings but are engaging fully in the process.”

Fujitsu declined to comment to the BBC.