The next leader of the SNP will need to be a far “more charismatic politician" than Humza Yousaf, Professor Sir John Curtice has said. 

In a scathing assessment of the outgoing First Minister, the pollster described the SNP leader - who announced his intention to resign on Monday morning - as "weak."

“The opposition has succeeded not in getting rid of a really popular highly charismatic, brilliant campaigning leader," he told The Herald.

"They have got rid of a weak leader who only just managed to win his party's contest.”

READ MORE: John Swinney considering bid to be next SNP leader

He had “very little charisma,” Prof Curtice added. “His speech today was awful. I mean, even Theresa May could do better than that.”

Whoever replaces the outgoing party chief will need to be a “more charismatic politician who is effective and can reach out to the wider public.”

But, he added, they will also need to be able to maintain the fragile coalition within the SNP.

So far only John Swinney has said he is actively considering a run at the top job. Sources close to Kate Forbes have suggested she too is thinking about it, though they say it is very early days. 

“Some of us remember John Swinney when he was SNP leader, and he wasn't really that good at it," Prof Curtice said.

“John as a deputy leader is absolutely brilliant because he could say nothing in more words than anybody else could ever string together. Fantastic at it.

“But what you need from a leader is an ability to say something really resonant in as few words as possible, and that's not John's skill.

“Kate Forbes, in contrast, does have the rhetorical ability, etc, etc. But can she create the coalition?

“I think she's probably now a much better politician than she was 12 months ago. She did eventually learn the art of saying things in code rather than necessary things explicitly, but a lot of people burned a lot of bridges with Kate Forbes last year."

READ MORE: Inside story: Humza Yousaf's resignation eases SNP's turmoil

Three others have been touted as possible successors - Education Secretary Jenny Gilruth, Wellbeing Economy minister, Màiri McAllan, and Health Secretary Neil Gray.

They are “a bunch of people who the public don't know" Prof Curtice said. 

“They do not have very much in the way of a wider public profile. And therefore it's a bit of an unknown quantity.”

Both Mr Gray and Ms Gilruth have let it be known that they will not stand and would back Mr Swinney. 

“I guess if Swinney decides to stand maybe nobody else will put their hat in the ring, in which case it might be a coronation, which is certainly better than a divisive leadership contest,” Prof Curtice added.

“But then Swinney has to steady what is still a difficult ship because there's still the police allegations hanging over [the SNP]. And he’s very much associated with the Sturgeon regime, etc, etc.

“But at least he's got the experience. At least he might make a reasonable fist of it, but they may still find themselves struggling.”

Stephen Flynn has also ruled himself out of the contest. He too has said he would encourage the former deputy first minister to stand. 

"I think there's only one person who can unite the party," he told the News Agents podcast. "I think there's only one person who can unite the country."

“Flynn is the best performer at PMQs at Westminster”, Prof Curtice said. “Flynn has many of the qualities that the SNP are looking for. 

“Maybe Swinney should need to think about a dual leadership whereby he's relying on Flynn to do a lot of campaigning - after all it will be Flynn and his colleagues who are trying to hang on to their seats in the short run - and he concentrates on trying to steady the ship north of the border.” 

“If I were touting to be SNP leader I'd want Flynn on my side,” he added.