Lord Advocate Dorothy Bain is coming under pressure to explain why the Crown Office and Procurator Fiscal Service (COPFS) took sub-postmasters in Scotland to court despite knowing there were problems with the Horizon IT software.

On Wednesday prosecutors said they had been made aware of problems in May 2013.

But documents obtained by the Scottish Conservatives showed that they knew on January 29 of that year.

The scandal of the faulty software which made it look as if hundreds of owner-operators in charge of sub-post offices had stolen money, dominated Thursday’s First Minister’s Question.

READ MORE: Horizon IT scandal: Scottish victims to be cleared

Labour leader Anas Sarwar used his questions to Humza Yousaf to highlight the case of Peter Worsfold, a sub-postmaster in Inverness between 1997 and 2002, who told the public inquiry of how Post Office employees came to his door “with menace” to demand money they wrongly believed they were due.

Mr Sarwar said it was “behaviour reminiscent of the mob.”

“These stories show that the Post Office behaved like a private police force and showed little regard for the law in Scotland.

“Sub-postmasters were pressured into accepting accusations of false accounting and forced to hand over thousands of pounds that day or face imprisonment.

“If any other organisation had behaved like this in Scotland we would expect to see criminal investigations into their conduct.”

The First Minister said he empathised in the "strongest way possible with the harrowing tales that we’ve heard from subpostmasters and subpostmistresses right up and down the country.”

He said the actions of the Post Office should be “interrogated” and said it is for the Crown Office to consider allegations of criminality.

The COPFS estimate that up to 100 people in Scotland have been convicted due to the faulty fujitsu software.

Unlike, England where the prosecutions were undertaken by the Post Office, the prosecutions here were dealt with by the Crown Office. 

READ MORE: Horizon IT scandal: Lib Dem leader Ed Davey under pressure

Mr Ross told MSPs that one Crown Office prosecutor dropped a case in January 2013, citing “issues with Horizon as reasons for not proceeding with a case.”

He raised comments from Stuart Munro, convener of the Law Society of Scotland’s criminal law committee, who said this week that the Procurator Fiscal should have gone public

"He says and this is his quote, 'the Procurator Fiscal has a legal duty to disclose relevant information of those accused of crimes and that duty continues even after a trial is concluded. As soon as the Procurator Fiscal became aware of concerns about the reliability of horizon that should have been disclosed."

He said the Crown Office had serious questions to answer.

“The Crown Office in Scotland must be transparent – prosecutors were aware of issues with the flawed Horizon system more than 10 years ago, so First Minister, we don’t need meetings or briefings from the Lord Advocate, we need her here in Parliament to answer questions about this scandal," he said. 

Responding, the First Minister said he had a conversation with Ms Bain on Thursday but that it is for her to decide if she should appear before MSPs.

Mr Yousaf said: “Scottish prosecutors were told in September 2013 to treat cases reported by the Post Office in regard to the facts and circumstances and evidence which did not rely upon Horizon.

“Then of course no cases were prosecuted from 2015 where the sufficiency of evidence was dependent on the evidence from the Horizon system.”

He said the Crown Office, between 2013 and 2015, had been assured by the Post Office that Horizon had no bearing on live Scottish cases.

Mr Sarwar said the Lord Advocate needed to lay out a timeline of the Crown Office’s understanding. 

READ MORE: Sunak announces new law to exonerate Post Office scandal victims

Rishi Sunak announced on Wednesday that the UK Government would bring in new legislation to quash the convictions of those convicted in England and Wales. 

Mr Yousaf promised that the Scottish Government would do similar in Scotland.

He said ministers would either use a legislative consent motion to effectively allow the UK Government to legislate in a devolved area or would develop separate legislation to deliver the same effect for Scotland.

Some of the victims of the scandal died before they could clear their names, and it has been linked to four suicides.

So far, only 16 people north of the border have asked the Scottish Criminal Cases Review Commission to look at their convictions.

Of those, just two have had convictions overturned.

Another four are currently appealing their convictions to the High Court of Justiciary Appeal Court, with decisions expected next month.

There has been a groundswell of public anger in recent days thanks to the ITV drama, Mr Bates Vs the Post Office, based on the stories of some of those wrongly pursued.