Humza Yousaf should have suspended Nicola Sturgeon after her arrest last month by police investigating the SNP's finances, according to the country's leading academic authority on the SNP.

James Mitchell, professor of public policy at Edinburgh University, said by not doing so Mr Yousaf looks "weak and indebted" to Ms Sturgeon.

His intervention comes as Mr Yousaf will mark 100 days this Friday since he was sworn in as first minister following his win in the SNP's leadership election which ended on the March 27.

"He looks weak and indebted to Sturgeon," Professor Mitchell told The Herald.

"Suspending the former leader would not only be consistent with how she behaved but also would have shown leadership and a willingness to be his own person. Sturgeon has not helped at all.  

READ MORE: Humza Yousaf: How long will he be SNP leader and First Minister?

"She should have resigned her membership on the understanding that this was so long as the enquiry continued. His loyalty to her has well outstripped her loyalty to him and the SNP. She has placed him in a very difficult position."

Ms Sturgeon was arrested on June 11 by officers investigating how more than £600,000 donated to the SNP by activists was spent.

She was the third person to be arrested in Police Scotland's Operation Branchform. Ms Sturgeon's husband Peter Murrell, the SNP's former chief executive, and Colin Beattie, the SNP's former treasurer were arrested in April. All three people were later released without charge pending further inquiries. In a statement following her release, Ms Sturgeon has said she was "innocent of any wrongdoing".

The day after her arrest Mr Yousaf said he saw "no reason" to suspend a party member who has been released without charge.

READ MORE: Ten dates in Humza Yousaf's time as SNP leader ahead of 100th day

He said: "I'll not suspend Nicola's membership. I'll treat her in the same way I've treated, for example, Colin Beattie.

"Those that have been released without charge I see no reason to suspend their membership."

He added: "She has been released without charge and I think it is so important that presumption of innocence is upheld."

His remarks came after Michelle Thomson, an SNP MSP who was forced to resign the whip when she was an MP eight years ago, said it would be consistent with the party’s previous actions if Ms Sturgeon voluntarily resigned the whip at Holyrood.

Ms Thomson, who was caught up in an investigation into alleged mortgage mis-selling but later cleared, said she was a strong believer in the presumption of innocence. Even so, the SNP’s rules were clear, she said.

Professor Mitchell said he recognised that while Mr Yousaf had been "dealt a difficult hand" with the party's internal problems and the policy delivery challenges the Scottish Government faced, it was up to public figures to "rise to the occasion".

He said: "Leadership requires senior public figures to rise to the occasion. Crises occur, major problems have to be confronted, dilemmas are unavoidable – these go with the territory.  

"He is unfortunate in facing these immediately on election, not having the luxury of a political honeymoon and facing the scale and breadth of unprecedented challenges.  

But while he may not have known about the problems with the SNP’s internal problems, he should have known that the SNP’s record in government was poor."

Professor Mitchell went on to say the SNP's inability to take advantage of the Brexit, the premierships of Boris Johnson and his successors would in the future "be seen as one of the greatest lost opportunities for the SNP in its history."

He told The Herald: "What [Humza Yousaf] inherited and what has happened in his first hundred days is unprecedented.  The SNP is facing its most serious crisis in decades with a police investigation, three senior members arrested and the need for an overhaul of governance and accountability.  

"What is becoming increasingly obvious to the public is that there is a gulf between its record in government and what it promised to deliver.  And it has made little advance in pursuit of its goal of independence despite favourable conditions. The inability to take advantage of the mess of Brexit, the premierships of Boris Johnson et al will, in the fullness of time, be seen as one of the greatest lost opportunities for the SNP in its history."