John Swinney is to leave the Scottish Government with Nicola Sturgeon, he has confirmed.

Mr Swinney, who has been the longest serving Scottish Deputy First Minister, described his almost 16 years in the Cabinet, first under Alex Salmond then under Ms Sturgeon, as the “privilege of my life”.

But he said after taking on several “demanding” roles, including as finance secretary, education secretary, and currently as the Covid Recovery Secretary, he will be stepping down from the Government when the new first minister is appointed.

In a letter to Ms Sturgeon setting out his intentions, Mr Swinney – who has been her Deputy First Minister since she took on the top job in 2014 – said he looks forward to serving with her on the backbenches of Holyrood “to continue our contribution to Scotland’s cause”.

He told Ms Sturgeon: “Thank-you for the opportunity you have given me to serve my country.

“I am indebted to you for your leadership, constancy, friendship and loyalty.”

Recalling how he joined the SNP at the age of 15 in 1979 at a time when the party’s prospects of electoral success were “poor”, he said he could “scarcely have imagined that over so many years I would have the opportunity to serve Scotland in Government in the way I have”.

Mr Swinney, who was the SNP leader between 2000 and 2004, thanked colleagues and also civil servants, praising Government officials for their “advice and professionalism”.

He said he had been “touched constantly by the kindness of people in Scotland and their understanding of the many challenges with which we have to wrestle in Government”.

He added he will remain at Holyrood as the MSP for Perthshire North, vowing to “faithfully represent” his constituents.

Ms Sturgeon, meanwhile, praised Mr Swinney for his years of service, telling him: “Your contribution to our nation, almost 16 years in Government is considerable, indeed unique. ”

She told him while she “completely” understood his decision to step down, she “felt a real sense of sadness”.

I could not have wished for a better partner in government than you, and there is no doubt that our Scottish Government would have achieved much less had you not been in it

She highlighted his role as finance secretary in the first SNP Government, when Mr Swinney ensured successive budgets got through Holyrood despite the SNP not having an overall majority.

In his time as education secretary, the First Minister said he had delivered the “biggest expansion in early learning and childcare in our country’s history” with the introduction of 1,140 hours a year of early years care for pre-school children, with Ms Sturgeon hailing this as a “transformation that will change the lives of future generations of children for the better”.

She also said Mr Swinney’s role in Scotland’s Covid recovery “merits special mention”, telling him: “Your friendship and wise counsel during the pandemic, which was by far the most challenging period of my tenure as First Minister, was invaluable and helped me through some really tough days.”

Ms Sturgeon added: “Over almost 16 years in government, more than eight years as Deputy First Minister – indeed, the longest serving Deputy First Minister so far – you have made countless contributions to the good of our people and our country.

“In short, I could not have wished for a better partner in government than you, and there is no doubt that our Scottish Government would have achieved much less had you not been in it.”

Mr Swinney’s decision to leave the Scottish Government comes as three candidates: Scottish Health Secretary Humza Yousaf; Finance Secretary Kate Forbes; and former community safety minister Ash Regan; are running to replace Ms Sturgeon as SNP leader and Scottish First Minister.

SNP members will choose their new leader in a ballot later this month, with voting closing on March 27.

Mr Yousaf said it had been “an absolute honour to serve in Cabinet alongside John Swinney”, hailing the Deputy First Minister as a “true giant of the SNP and the independence movement”.

He said: “Our party and our Government owe much of our success to John’s hard work, from securing the council tax freeze in the early days of Government, to expanding childcare as education secretary, finding the funds to help households through the cost of living crisis and enabling me to deliver fair pay for our NHS in recent weeks.”

Meanwhile, Ms Forbes spoke of her “respect and admiration” for Mr Swinney, who took responsibility for public finances for her while she was on maternity leave.

She said: “John Swinney is not only an incredibly able politician, who has put public service at the heart of his career, he’s also somebody who I’d call a friend and is a friend to so many MSPs.

“I have valued enormously his advice and guidance over the years and I wish him well because he has put in some shift.”