Top civil servant warns Boris Johnson's 'assertive unionism' will lead to independence

He also said Boris Johnson's government was showing signs of adopting an 'assertive unionism' strategy towards Scotland, which differs from the approach taken by his predecessors in trying to develop a collaborative partnership between governments.

It comes after a hard-hitting report by University of Cambridge's Bennett Institute for Public Policy, co-authored by Mr Rycroft, was published last week.

It suggested that Boris Johnson's dislike of devolution and Whitehall's 'indifference' to devolved governments was a serious threat to the union.

The UK Government, and Boris Johnson himself, have insisted that devolution and keeping the United Kingdom together is one of their key aims, however Mr Rycroft has suggested they may be going about it in the wrong way.

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He told the Herald on Sunday: “This government was elected on a hard Brexit ticket, whose whole motivation has been driven by this restoration of sovereignty to Westminster.

“You're not going to find there is a well of empathy for the devolved governments because, of course, those devolved governments have their own mandates, democratically elected.

“Around the cabinet table you don't have that many folk who have that instinctive sympathy for the devolved process.

“On top of that there is a Prime Minister who is prepared to say, albeit maybe off the record, to his MPs that devolution had been a disaster.

“He's a man who is prone to speaking before he thinks.”

Mr Rycroft said that the prime Minister's comments, made to around 60 MPs from the north of England during a Zoom call, could be a sign of a shift within Conservative party thinking around devolution and how helpful it has been.

Mr Johnson's remarks emerged on November 16 last year, where he is reported to have said that devolution had been “a disaster” and “Tony Blair's biggest mistake”.

Four days later the Prime Minister told his MPs that he was not against devolution, but had been referring to “the way the SNP has handled devolution in Scotland.”

He said at the time: “Just because I have criticised the performance of devolution does not mean I want to oppose devolution as a concept.”

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Asked if the view that devolution had been a “disaster” was something growing in popularity within the conservatives, Mr Rycroft said: “There is a possibility. I don't know for certain, but there is a strain of thinking around the Conservative party, certainly not the whole of the party but sections of it, that devolution was a mistake and that the way to keeps Scotland in the union is through so-called 'assertive unionism', where you take this hard-line approach against the SNP, very aggressive against the Scottish Government and you think of devolution as the problem, not the solution.

“Certainly if he has been hearing that sort of rhetoric, that is possibly why that thought was close to the surface of his mind.

“You'd never have heard that equivalent from Theresa May or David Cameron. They wouldn't have been in that mindset.”

The former top cabinet office official said there had “always been people who believed assertive unionism was the right way forward, but they certainly haven't been in the ascendance.”

This is evidenced, he said, by the introduction of the Scotland Act 2016 and the Wales Act 2017 by David Cameron.

He explained: “Secretaries of state in place at the time would have been well behind that. It was about making changes to the devolution settlement in terms of tax and welfare, and them demonstrating their respect for the role of the Scottish Parliament and Welsh Senedd, and demonstrating that they understood the devolution process and were prepared to take it further.

“That was the predominant strain of thinking certainly through the cameron era, then when Theresa May came in Brexit dominated everything. So this has really only emerged as visible in government policy under Boris Johnson.”

As detailed in the University of Cambridge report released on Monday, and alluded to in the long-awaited Dunlop Report commissioned by Theresa May, Whitehall officials have a lack of understanding, or an 'indifference' to devolution, which has to be addressed for the union to remain together.

Mr Rycroft explained: “The mindset in Whitehall has been not really getting how the devolved parts of the UK function.

“What people crave is respect. There may be differences, but they want to know that at least they are respected.

“ But what we're seeing in things like the imposition of the UK Internal Market Bill, the way they are going to distribute funding in the shared prosperity fund and so on, it's disrespectful to say 'You've elected a government but we're going to ignore them'. Why have this antagonistic approach?

“This is a union of four parts and whatever the theory of parliamentary sovereignty, this is a voluntary union.

“This isn't about dissing the SNP, it's about persuading the majority of people in Scotland that this union has a future . You have to play your cards differently if you are going to persuade people to make that emotional commitment to the future of the union. This is why I think a lot of us are just puzzled by the approach the UK Government has taken.”

Asked if he thought Boris Johnson did want Scotland to remain part of the UK Mr Rycroft said: “I can only go by what he says publicly and he, like all previous prime ministers has said, he is very committed to sustaining the union and indeed his political future in many ways rests on that.

“If the union breaks up, in my view it's bigger than Brexit. No prime minister wants to be the one who presides over the break-up of the territory, so whatever his personal views his political fortunes really rest on sustaining the union.

A UK government spokesman said:“The United Kingdom is the most successful political and economic union the world has ever seen, and this pandemic and our collective response, from the furlough scheme to vaccine procurement and the backing of our military personnel, has shown that we are at our strongest when we work together towards a common goal.

"Strengthening the United Kingdom is at the heart of everything we do and we are working alongside the devolved administrations to establish new ways of regular, meaningful and effective cooperation so that we continue to deliver for people right across the United Kingdom"