Alan Bates, who has led the campaign to rectify what has been called the UK’s biggest miscarriage of justice, has accused the Post Office of spending two decades “denying, lying, defending and attempting to discredit and silence me”.

He also described the response of Ed Davey, a former Post Office minister and now leader of the Liberal Democrats, to his appeals for help as “disappointing and offensive”.

“[The government] should have been involved far earlier,” Bates said of the conduct of the Post Office’s sole shareholder while giving evidence on Tuesday morning.

In his first appearance at a public inquiry into the scandal, Bates, 69, whose battle over the injustice was the focus of a celebrated ITV drama, laid out the events that led to his campaign, including the termination of his own contract in 2003.

Bates, who took over his Post Office branch in Llandudno in 1998, told how he had repeatedly complained during his tenure that the Horizon accounting system could not be relied upon and that it was wrong that operators were being obliged to make good on shortfalls.

His contract was terminated without any reason being given in November 2003.

To laughter in the inquiry room, Bates was then shown internal documents unearthed by the inquiry’s lawyers in which Bates’s termination was said to be due to him being “unmanageable” and he was referred to as someone who “struggled with accounting”.

Speaking at an inquiry session in Aldwych house in London, Bates responded: “It’s just they decided they were going to make a lesson of me.” He added that his determination to uncover the faults of the Horizon scandal was due to “stubbornness” and a sense of injustice after learning that hundreds of others had also been affected.

“I didn’t set out to spend 20 years doing this,” Bates said. “Once I’d started my individual little campaign and we found others along the way, and eventually we all joined up. It has required dedication, but secondly, it is a cause.

“I think it’s also stubbornness as well. But it’s … I mean, as you got to meet people, and realise it wasn’t just yourself. And you saw the harm, the injustice that had been descended upon them, it was something that you felt you had to deal with.”

Between 1999 and 2015 hundreds of post office operators were accused and in some cases convicted of crimes relating to theft, false accounting and fraud, based on faulty information from the Horizon computer system which erroneously suggested that money had gone missing from branch accounts.

Bates’s own experience began just three months after the system had been installed in his branch, when a £6,000 shortfall emerged. Bates discovered the fault that could account for much of the money but there remained a £1041.86 loss. The Post Office agreed to write that sum off but a line manager complained when Bates refused to make good on further paper losses.

Bates said he did not trust the Horizon system and that he was not able to interrogate the system himself to find where the faults lay. He launched a campaign with other victims but for years their calls for an inquiry were ignored by government.

Davey, who was Post Office minister between 2010 and 2012, initially refused to meet Bates. He met him and his lawyers only after civil servants warned of a presentational risk if he failed to do so at a time when Channel 4 was planning a documentary over the affair.

Davey was advised by civil servants to avoid offering any sort of investigation into the scandal, documents seen by the inquiry disclosed.

The briefing to the minister said: “Demonstrate you’re prepared to hear their side of the story. But make it clear you’re not in a position to offer substantive comment and avoid committing to setting up an independent or external review of Horizon.”

Bates said nothing positive came from the discussion.

The inquiry also heard that the Post Office was continuing to disclose thousands of documents to the inquiry at late notice after failing to conduct an earlier review of emails and other correspondence handled by personal assistants of key witnesses, including the former chief executive Paula Vennells, who is due to give evidence next month.

Jason Beer KC, the inquiry’s lead counsel, said that since early February the Post Office had disclosed nearly 70,000 documents to the inquiry, that late disclosure had caused the inquiry “much more than minor or ad hoc” issues, and that the organisation’s method of handing over important emails and data was “very concerning”.