Kate Forbes has said she will support Scotland’s new First Minister despite rebuffing the offer to serve in the cabinet.

Ms Forbes, who served as Finance Secretary under Nicola Sturgeon’s administration, is believed to have been offered the role of cabinet secretary for rural affairs and islands by new First Minister Humza Yousaf, but turned down the position.

It means the 32-year-old, who served as Finance Secretary under Nicola Sturgeon’s administration, will leave the Scottish Government and return to Holyrood’s backbenches.

If she had moved to rural affairs it would have been seen as a major demotion, one which Forbes backer and former health secretary Alex Neil branded an “insult and not a real effort to unite”, adding: “A poor start.”

Mr Yousaf was named SNP leader on Monday, when it was revealed that he won 52.1% of the vote once he progressed to the second round.

Ms Forbes narrowly missed out on the leadership, claiming 47.9% of the vote in the second round, while Ash Regan was eliminated in the first round with 11% of the vote.

She said on Tuesday that she was “delighted” to have voted for Mr Yousaf to become First Minister when the majority of MSPs backed him for the top job.

“He has my full support, as he governs well and furthers the case for independence,” she tweeted.

“I have full confidence he will appoint a talented cabinet and ministerial team, able to meet the challenges facing the country.”

She tweeted that it was to the “great credit” of the new First Minister that the first official conversation he had after the result was with her.

“He has been respectful, supportive and warm through. In whatever capacity I serve, I’ll support him,” she wrote.

But during the almost six-week-long SNP leadership campaign, she had launched a series of attacks on Mr Yousaf and criticised his record in government.

In one debate, broadcast on STV, she told him: “When you were transport minister the trains were never on time, when you were justice minister the police were strained to breaking point, and now as health minister we’ve got record high waiting times.”

But she too came under fire from SNP colleagues over her socially conservative views, and the Scottish Greens, who agreed the Bute House powersharing deal with the SNP, had signalled that they would not be likely to work with Ms Forbes if she were elected as the country’s next first minister.

Also on Tuesday evening, MSP Ivan McKee, the business minister who was heavily involved in Ms Forbes’ campaign, was reported by The Times as likely to quit ministerial office after being offered a similar position with fewer responsibilities.

The Scottish Conservatives have said Mr Yousaf has “poured petrol on the SNP civil war” on day one of being First Minister.

MSP Craig Hoy, chairman of the Scottish Conservatives, said Ms Forbes’ “furious snub to his offer of a cabinet demotion shows his mission to reunite their feuding party is doomed to failure”.

“The reality is the SNP are divided from top to bottom. And, as they concentrate on knocking lumps out of each other, they are unable to focus on the real priorities of the Scottish people,” he said.