Swinney raised concerns over Sturgeon's independence strategy

John Swinney was "nervous" about Nicola Sturgeon's 2016 push for a second referendum

John Swinney raised concerns with Nicola Sturgeon about her push for a second independence referendum in the wake of the Brexit vote, he has revealed in a new documentary.

Ms Sturgeon said a fresh ballot on independence was “highly likely” after the UK voted to leave the EU in 2016.

But Mr Swinney says he was “nervous” about how the SNP could motivate people to support the move so soon after the vote in 2014.

The party ultimately lost 21 seats in the snap election of 2017, with Mr Swinney saying the referendum pledge went down "very badly" with voters.

Ms Sturgeon admitted that it was “out of step with the public mood”.

Mr Swinney now leads the SNP, and has said the party needs to reflect on how it approaches the independence campaign in light of further losses in July’s general election.

Nicola Sturgeon said an independence vote was "highly likely" after Brexit

The two-part documentary charts the fortunes of the SNP under the leadership of Ms Sturgeon and her predecessor as first minister, Alex Salmond, and the disintegration of their personal relationship.

In it, Mr Swinney reveals that he raised concerns with his then-boss, Ms Sturgeon, about her response to the EU referendum in 2016.

A majority of Scottish voters backed Remain while the UK as a whole voted to Leave, which Ms Sturgeon said meant a fresh contest on independence was "on the table".

Mr Swinney said: "I remember saying to Nicola as she was formulating the message she was going to convey in the aftermath of the Brexit referendum, was she sure she wanted to say so firmly that she was opening up dialogue about a further referendum?

"I have to say I was nervous because I was still concerned by how we were able to motivate people in Scotland when we had just had one in 2014."

John Swinney spent years as Nicola Sturgeon's right-hand man in government

Ms Sturgeon said the SNP had "very deliberately" cited Brexit in its 2016 Scottish Parliament election manifesto as something which could create a "material change of circumstances" that could justify returning to the issue of independence.

She said there was "a lot of debate" within her core team about it - including "a number of occasions" where she spoke to her predecessor, Mr Salmond, who had urged her to "pick up the pace and be more decisive".

However, she added: "My confidence levels going into the 2017 election were not at their highest.

"I was reasonably confident I had set us on the right course, but it was a course that was at that point out of step with the public mood."

Mr Swinney said that during the snap election called by Theresa May, "the referendum question was playing very badly for us politically, and it was eroding support".

The SNP ultimately lost 21 seats in the contest - with the biggest scalp being that of Mr Salmond, who was defeated by the Conservatives.

And Ms Sturgeon's call for a referendum was rejected by a series of Tory prime ministers, on the basis that the issue was recently settled in 2014.

Mr Swinney, Ms Sturgeon and Mr Salmond were once a close political team

Following Mr Salmond's exit from Westminster, the relationship with his successor as first minister deteriorated.

The documentary notes that there had already been tension after Mr Salmond urged Ms Sturgeon to drop her husband, Peter Murrell, as the SNP's chief executive.

Ms Sturgeon said she had tried to call her former mentor on several occasions in the weeks after the he lost his seat, but that he had not replied.

Mr Salmond - who now leads the rival Alba Party - told the documentary that he did not rate Ms Sturgeon's team or the advice she was getting.

He said: "Nicola started out by demanding a referendum, by taking the troops up to the top of the hill, and then when that didn’t seem to meet with public approval she marched them back down again.

"It’s one thing to not think you can achieve the referendum that had been promised in successive polls, it’s another to keep announcing you’re going to do it and then not doing it, and that has a demoralising effect."

The issue remains a topical one with Mr Swinney now in charge of the SNP's independence campaign, having suffered another damaging electoral reverse.

After the SNP lost 39 seats in July's UK poll, he said the party had "failed to convince people of the urgency of independence", and that it must "take time to consider and reflect".

And at its Autumn conference, he was recorded telling delegates that the SNP had spent too long focusing on the "process of independence".

The first episode of “Salmond and Sturgeon - A Troubled Union” will air at 22:00 on Tuesday, 10 September, on the BBC Scotland channel.