Richard Leonard has called on Nicola Sturgeon to refer herself to the panel of independent advisers on the ministerial code over her meetings with Alex Salmond.
The First Minister said that she met Mr Salmond on three occasions and had spoken to him on the phone twice after sexual harassment allegations had been made by two women against the former First Minister to the Scottish Government in January 2018.
During First Minister’s Questions on Thursday, opposition leaders questioned why Ms Sturgeon met Mr Salmond after the allegations were made and asked why no minutes were taken.
Ms Sturgeon told MSPs: “It seems to me that I am being simultaneously accused of being involved in a conspiracy against Alex Salmond, and also of colluding with Alex Salmond.
“Nothing could be further from the truth in both of those – neither of those things are true.
“Since I found out about the investigation I have tried to do the right thing in a situation which, no matter what happened, was never going to be easy for me.
“And the most important thing here has always been and continues to be the complaints that were made and the people who made those complaints.”
Mr Salmond strongly denies the allegations, which are the subject of ongoing police inquiries.
Speaking on the BBC’s Good Morning Scotland on Friday, Mr Leonard, the Scottish Labour leader, said he believed that Ms Sturgeon had breached the ministerial code.
Mr Leonard said: “I think we’re talking about days, rather than weeks. She has said in the statement that she made earlier in the week that she has to pause and reflect on things.
“I think that in light of the evidence which came out yesterday, in First Minister’s Questions, that the momentum is growing ever greater for her to self-refer to the independent advisers on the ministerial code.
“If she self-refers, then that starts that process off. If she refuses to self-refer, then I think Parliament will try and take matters into its own hands.
“But again, that’s contingent on us winning support from other parties and other MSPs in the Parliament, which I hope that we can do.”
Mr Leonard said that if the First Minister decides against making a self-referral, the Scottish Parliament could establish a committee, or use an existing committee to probe into whether there had been a breach of the ministerial code.
He said: “The idea that she could meet with the former First Minister of the Scottish Government, as the current First Minister of the Scottish Government, and discuss a Scottish Government investigation which is being considered because of complaints made by two Scottish Government employees, is clearly a Governmental conversation that she had and ought to have been properly recorded according to the code.”
Meanwhile Scottish Conservative interim leader Jackson Carlaw repeated his calls for Holyrood to mount an inquiry.
The Tory said: “As we have seen this week, Nicola Sturgeon’s case simply doesn’t stack up.
“She claims she told Mr Salmond in April last year that she would not get involved in this matter. If that was the case, why did she carry on taking phone calls and having meetings with him to discuss the matter?”
He said Ms Sturgeon in particular “needs to come clean on her bizarre decision to hold a Saturday summit with Mr Salmond at her home in July”, saying that as she had already spoken to the former first minister several times by then, she needed to explain the reason for the meeting.
Mr Carlaw claimed that “failure to answer these questions make a Holyrood inquiry inevitable”, as he said: “The SNP cannot continue to sweep this matter under the carpet and patronise the public by claiming it’s all a matter for the SNP.
“It isn’t. Two Scottish Government employees have been let down, £500,000 of taxpayers’ money has been wasted, and the SNP administration’s competence is up for question.
“We need to know what has gone on – the First Minister must front up immediately.”