It is a scandal that has rumbled for years, and yet it is suddenly dominating UK politics. Here is a guide to the saga of the Post Office and its Horizon IT system.

What is it all about?

It has been described without exaggeration by the British prime minister, Rishi Sunak, as one of the greatest miscarriages of justice in UK history: the hounding and prosecution of thousands of people who owned and ran smaller post offices for alleged fraud between 1999 and 2015, the overwhelming majority of whom were falsely accused.

The initial fault was with Horizon, a digital accounting system installed by the IT multinational Fujitsu, which wrongly said post office branches had cash shortfalls. This was then compounded by the Post Office, the company behind the network, which rejected any fault with Horizon and insisted operators must have taken the money, to the extent of covering up the real problem.

Overall, 3,500 branch owner-operators were wrongly accused and more than 900 prosecuted, with many of these jailed and ruined. Some suffered significant ill health, local ostracism and family breakup, and in a handful of cases, suicide.

Who were the people wrongly accused?

These were, in the traditional job description, subpostmasters and postmistresses, who owned and ran smaller post offices as franchises.

While they were independent, often owning the building where the business was based, they were part of the Post Office system, which handles not just letters and parcels but services including banking, bill payments, money transfers and applications for documents.

The Post Office, which has about 11,500 branches across the UK, was formerly part of Royal Mail, but was split off in 2012 when the mail service was privatised.

How could the injustice last so long?

This is one of the key questions for a public inquiry into the scandal that began in 2021 and is still hearing evidence, but the main driver was seemingly a toxic and secretive management culture in the Post Office, with the victims marginalised, dismissed and disbelieved.

Numerous owner-operators accused of theft were told they were the only people facing such claims, only to find out later that hundreds of others had been similarly targeted when some of those wrongly targeted raised the alarm. Notable among these was Alan Bates, a post office operator accused of stealing thousands of pounds from his branch, who set up a campaign group.

Aware something could be wrong, the Post Office even commissioned a company of forensic accountants to look into the fraud claims, but after these found possible fault with Horizon, their contract was terminated.

The Post Office was also subject to government and official oversight, and there is much debate about whether ministers and civil servants could have addressed the scandal sooner.

When was the injustice acknowledged and redressed?

It took a long time. It was only in 2019 that a group of post office operators won a high court case ruling that their convictions were wrongful, with Horizon at fault. This decision was upheld on appeal in 2021, quashing the convictions of some workers and beginning the road to compensation.

Between these two court decisions, in October 2020, the Post Office formally apologised for what it called “historical failings”.

However, the process of clearing those affected had been slow: by December, 142 appeal case reviews had been completed out of 900-plus people convicted.

Why is the story suddenly in the headlines now?

For one reason: Mr Bates vs the Post Office. The four-part drama broadcast in the first week of January by ITV, the UK’s main commercial TV station, starred the actor Toby Jones as Alan Bates. It powerfully brought home the human consequences of the saga, bringing an immediate reaction from the media and politicians.

What happened in response to the drama?

Less than a week after the final episode was aired, Sunak announced an unprecedented plan to pass a law that would overturn the convictions of all those accused over Horizon-related fraud or theft, and offer them swift compensation – either an agreed lump sum of up to £600,000 or an amount to be agreed. The plan is for this to be completed by the summer.

Is this the end of the story?

No. Some politicians connected to the scandal have faced calls to apologise, and there are suggestions that Fujitsu should cover the costs of the compensation. All this is likely to have to wait until the public inquiry reaches its conclusions, at a date as yet unknown. The Post Office is now itself under criminal investigation over potential fraud offences.

Paula Vennells, the former Post Office chief executive, has already said she will return an official honour she received.

Some MPs from Sunak’s Conservative party have focused criticism on Ed Davey, calling for him to quit as leader of the smaller opposition Liberal Democrats for, they say, not taking the scandal seriously when he was the business minister responsible for the Post Office from 2010-12. The Lib Dems call the attacks partisan and unfair.