Humza Yousaf's election as SNP leader is the “main culprit” in the slump in support for the SNP, according to the country’s top pollster.

Professor Sir John Curtice of the University of Strathclyde, said that the past year had been the most challenging for the party since it took office at Holyrood in 2007.

In an analysis, he wrote the resignation and arrest of Nicola Sturgeon and her husband Peter Murrell, who have since been released without charge, was not the main catalyst for the steep decline in the SNP’s poll ratings.

“Twelve months ago, the party stood on average at 45 per cent in polls of voting intention for Westminster — in line with the party’s performance in the 2019 UK election,” he wrote on the website Bylines Scotland.

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“Although Labour had overtaken the Conservatives as the second most popular party north of the border, it was still as much as 21 points behind and seemed no more than a distant threat.

“Yet in polls taken since the arrest of Sturgeon at the beginning of June, support for the SNP is down to 36 per cent — as weak a position as it has been at any stage since the 2014 referendum.”

Last month a survey published in The Sunday Times suggested that Labour was poised to overtake the SNP at the next Westminster election for the first time since 2010.

The Panelbase Poll indicated that Sir Keir Starmer’s party was in line to win 26 seats, a big rise from the one they hold now. If replicated at an election the SNP would lose more than half of their 45 seats.

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Sir John claimed that projections for Holyrood suggest the SNP and Greens were in danger of losing their pro-independence majority in the chamber.

“Support for the SNP did ease a little to 43 per cent in the wake of the row about gender recognition, but no more than that, and was the level at which it stood when Sturgeon announced her resignation,” he said.

“Meanwhile, the arrest of Peter Murrell and the widespread publicity of the SNP’s financial difficulties to which it gave rise only saw a slight easing of SNP support – at least until the arrest of Sturgeon, which was followed by a two-point fall in SNP support to its current level of 36 per cent.

“That leaves one potential main culprit for the SNP’s current difficulties — the SNP leadership contest. It started with the SNP at 43 per cent in the polls. By the time the contest was coming to a conclusion, it had slipped to just below 40 per cent.

“Meanwhile, once Yousaf had been declared the winner it had slipped to between 38 per cent and 39 per cent.

“The election of a new party leader usually gives their party a boost in the polls – but of this, there was no sign at all following Yousaf’s success.”

He said that Mr Yousaf had a “relative lack of popularity” related to a lack of “charisma at the top that was provided by Salmond and Sturgeon”.

Mr Yousaf won the SNP election contest in April defeating second placed Kate Forbes, the former finance secretary, by 52% to 48% in the second round of voting. Former community justice minister Ash Regan came third.

During the campaign he was regarded as Nicola Sturgeon's choice as successor which was seen as giving him an advantage over his rivals.

In response, the SNP pointed to a recent poll, by Redfield & Wilton Strategies, which showed that support for the party had risen by two points to 37 per cent at Westminster, and by three points in the Holyrood constituency vote to 36 per cent, putting it ahead of Labour.

Keith Brown, the party’s depute leader, said: “This poll shows that people across Scotland continue to back the SNP and our positive vision for Scotland.”