What is the Post Office Horizon bill and why is it controversial?

Bill is unprecedented in its aim to exonerate post office operators in England and Wales en masse

• Ministers introduce bill to quash convictions of Horizon scandal victims

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.