The Post Office (Horizon system) offences bill, published on Wednesday, will quash the convictions of people resulting from the scandal, which involved hundreds being wrongly prosecuted on the basis of a faulty IT system. Here the Guardian explains the thinking behind the bill and why it is controversial.

What does the bill do?

It quashes all relevant convictions of offences such as theft, fraud and false accounting allegedly committed between 1996 and 2018 by people working in post offices in England and Wales that used the Horizon system software. People who have already had their case considered by the court of appeal will not be eligible.

How is it unprecedented?

To have a conviction overturned, people must normally apply to the Criminal Cases Review Commission, which will refer it to the court of appeal if satisfied that there is a real possibility the court will agree there has been a miscarriage of justice. Only the court of appeal can quash the conviction. There is no existing legislation that allows convictions to be referred or quashed en masse.

Why did the government take this approach?

The ITV drama Mr Bates vs the Post Office put the injustice in the spotlight, prompting calls for it to be addressed more speedily. This was compounded by reporting about ministers’ historical failure to take action and claims that they had sought to delay compensation payments. There were concerns that given the backlog in the courts, itself a consequence of underfunding, it could take years for individual cases to be resolved. As well as the injustice of the convictions themselves, people are not eligible for compensation until they are overturned.

Why is the bill controversial?

Under the rule of law, parliament makes the law and the courts carry it out. By legislating to overturn convictions, parliament is interfering with judicial independence and setting a dangerous precedent, legal figures have warned. Appearing before parliament’s justice committee, the lady chief justice, Dame Sue Carr, said it was untrue that the judiciary would not be able to deal with the volume of outstanding cases arising from the scandal and also untrue that she had given the legislation the green light. When subsequently asked about the matter at her first annual press conference, she stressed that it was “for the courts to make judicial decisions”.

By quashing all convictions within the relevant period, some guilty people could also be exonerated and receive compensation, which has offended the sense of justice of not just lawyers but some of the victims too. To try to – at least partly – combat this, the government said that, before receiving compensation, post office operators would be required to sign a legal statement vowing that they were innocent, at the risk of being prosecuted for fraud if they lie.