Deputy First Minister John Swinney has revealed he is backing Humza Yousaf to be Scotland’s next first minister - becoming the most senior SNP politician to support the health secretary.

Mr Swinney said Mr Yousaf was the candidate who could “strengthen the SNP as a force for progressive change in Scottish politics”.

He is the latest member of the Scottish Government to endorse Mr Yousaf and follows support given to his bid for the top job from a range of other senior party figures including constitution secretary Angus Robertson, education secretary Shirley-Anne Somerville and social justice secretary Shona Robison.

Ms Forbes has the backing of business minister Ivan McKee as well as a number of backbench MSPs including Christine Grahame, Jim Fairlie and Michelle Thomson.

The third candidate Ash Regan has been endorsed by the prominent SNP MP Joanna Cherry.

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Mr Yousaf hailed Mr Swinney's backing as a “massive boost” for his campaign, while Ms Forbes's camp said the endorsement - just ahead of the ballot opening on Monday - suggested Mr Yousaf's team was "absolutely panicking".

The health secretary said: “I am honoured to have the backing of a true giant of the SNP and independence movement, John Swinney, to become SNP leader.”

Mr Swinney, who is Scotland’s longest serving Deputy First Minister, is stepping down after more than eight years in the post and will leave office later this month when Ms Sturgeon steps down.

She has already made clear she will not publicly back any of the three contestants seeking to replace her.

Endorsing his cabinet colleague Mr Yousaf, Mr Swinney spoke about how he was “proud” of what the SNP has achieved since coming into power in 2007, adding this was “transforming the lives of people in Scotland”.

He added: “As a party we have never been closer to winning independence.

“We now need to choose an SNP leader who will complete our journey to independence, and I believe that person should be Humza Yousaf.

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“Humza is best placed to lead our party because he will strengthen the SNP as a force for progressive change in Scottish politics.”

With Mr Yousaf having backed reforms to the gender recognition process - and also vowing to challenge Westminster’s veto on laws passed by Holyrood on this in court - the Deputy First Minister said Mr Yousaf would “govern effectively by using the partnership that we enjoy with the Scottish Green Party, guaranteeing us a pro-independence majority in the Scottish Parliament”.

He also said that “crucially”, Mr Yousaf would be able to widen SNP support by “attracting new supporters to Scottish independence”.

Mr Swinney added: “Humza is an experienced minister who’s done all the tough stuff that you have to do day in, day out, within government. He’s put his heart and soul into all the work he’s done within the party to make the SNP an inclusive political party. So for me, John Swinney, it has to be Humza.”

Mr Yousaf stated: “Like Nicola, our party owes so much of our success to John’s stewardship.

“John’s backing is a massive boost to my progressive grassroots campaign to stand up for Scottish democracy, protect and advance rights, unite our Yes movement and secure independence for Scotland.”

Michelle Thomson MSP, who is backing Ms Forbes, said: "When the current Deputy First Minister decides to personally intervene in the leadership contest at such a late stage, you get the sense that senior figures in the party have seen the polls and are absolutely panicking.

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"Given how close we are to the vote opening, many party members will look upon this 11th hour intervention rather cynically. They don't need to be told by party HQ what to think or how to vote.

"It increasingly looks like the top brass would much prefer an establishment transfer of power - to carry on with the status quo - rather than allowing our membership the freedom to make a healthy democratic choice.

"I certainly trust our members to be independent-minded and firmly believe they can make their own minds up on who best serves the party, the country and - crucially - can deliver independence for Scotland. There is no doubt in my mind that that leader is Kate Forbes."

A poll last week suggested Ms Forbes was the public’s favourite to become the next First Minister of Scotland.

The Ipsos survey, commissioned on behalf of Channel 4 News, found the finance secretary had an eight-point lead against her closest rival Mr Yousaf.

Asked who they thought would make the best First Minister based on what they had heard so far, a third (32%) of the Scottish public backed Ms Forbes.

Mr Yousaf came in second with nearly a quarter (24%) supporting him while fellow SNP leadership hopeful Ash Regan fell in at third place with 8% of Scots backing her.

Some 24% of respondents said none of the candidates would make a good First Minister while 11% said they did not know.

The poll found Yousaf and Forbes were neck and neck among 2021 SNP voters, with 33% backing the health secretary and 32% backing the finance secretary.

The poll of 1503 Scottish adults was conducted between March 6-7 before the STV leadership debate.