Stephen Flynn said that Ms Bain should explain why prosecutions occurred after concerns over the Horizon system emerged in 2013.

She is expected to appear before MSPs at Holyrood this week to face scrutiny over the scandal.

Speaking to BBC Scotland’s Sunday Show, Mr Flynn said: “As I understand it, since 2015 at the behest of the Crown there were no prosecutions taken forward where evidence was reliant upon the Post Office saying it was a consequence of the Horizon system.”

He added: “I think between 2013 and 2015, there was of course not that clarity provided, and I think it’s incumbent upon the Lord Advocate – and I think the suggestion at FMQS [First Minister’s Questions] earlier in the week was to this effect – that the Lord Advocate outlines why exactly that was the case.”

The news comes as Lord Cameron said he could not recall being briefed about the scandal during his time as prime minister.

He told Sky’s Sunday Morning With Trevor Phillips: “I don’t recall in any detail being briefed or being aware of the scale of this issue.

“We’ve got to get to the bottom of it, and that’s what inquiries do, they can look at what were ministers told, what information was put in front of them, what did they decide – because it’s hard to remember every letter and piece of paper put in front of you.”