The Tory cabinet minister declared claims of a near “breakdown” in relations between Holyrood and Westminster were a deliberate ploy to undermine the UK.

He spoke out in an article for The Scotsman today after a report by the Scottish affairs committee (SAC) last week suggested the Scotland Office could be axed.

The committee’s chairman, Pete Wishart, insisted yesterday that Holyrood ministers now bypass the Scotland Office and deal directly with their Westminster counterparts as he stood by calls for a review of the system.

Scottish Secretary David Mundell delivers a speech to mark the 20th anniversary of the establishment of the Scottish Parliament at Dovecot Studios in Edinburgh.

A new “Department for the Union” could be established, the report last week suggested, replacing the three territorial departments which cover Scotland, Wales and Northern Ireland. But Mr Mundell has issued a staunch defence of his role today, dismissing claims of a “collapse” in relations as a ruse by Nationalists seeking to attack the UK.

Insisting his department represented Scotland’s interest at the heart of government and “strengthens Scotland’s place in the UK”, Mr Mundell said: “The Union is under threat. Making the case for Scotland’s place in the UK must remain as big a priority for the UK government under a new prime minister as it is under Theresa May’s premiership.

“So the Scotland Office must also challenge misleading claims that are designed to undermine the Union – claims about a ‘Westminster power grab’ as the UK leaves the EU, for example, or of relations between Scotland’s two governments breaking down.

“Those old myths resurfaced in the comment and commentary surrounding the SAC report and need to be called out.”

He insisted the Scotland Office provide “detailed evidence” setting out the range of engagement between the UK and Scottish governments.

“It demonstrates a remarkable degree of joint working, at ministerial and official level, on everything from Brexit preparations to agriculture, from city deals to the transfer of Holyrood’s new welfare powers,” he said.

Relations between the UK government and devolved administrations reached a low point over the future of powers in devolved areas held by the EU, which will be returned after Brexit.

The Scottish and Welsh governments claimed that plans for Whitehall to keep control of a handful of powers in key areas like agriculture, fisheries and the environment represented a “power grab” that undermined the devolution settlement.

Mr Mundell insisted such spats did not mean the system was flawed, just that Tory ministers “don’t see eye to eye with Nicola Sturgeon”.

But Mr Wishart insisted there were serious issues over engagement between the two governments.

“What we concluded was that relationships have almost broken down,” he told BBC Politics Scotland.

“We know that the Cabinet Office are currently looking at this.”

He added: “I think the Scotland Office and the role of Secretary of State has not responded to the evolution of devolution in the course of the past 20 years.

“What’s happening is that most Scottish Government ministers now go direct to their counterparts at Whitehall and miss out the Scotland Office.”

Mr Mundell’s department has a “limited role” in the day to day relationships between Scottish Government ministers and their Whitehall counterparts, Mr Wishart said.

“What we’ve suggested, and it’s only a suggestion as this goes forward, is that a review needs to happen to the Scotland Office and the proposal we’ve suggested is that we do have this Department of Constitutional Affairs, which brings together all the departments across the UK into one department,” he said.

Labour MP Paul Sweeney said a structure was needed that worked regardless of which parties were in power at Westminster and Holyrood.

“What’s clear is that there has been a lack of codification of the processes of how different departments in Edinburgh and London work,” he said.

New PM has to defend a union under threat, says David Mundell