The UK Government must adopt a permanent “campaign mode” with a war-chest running into tens of millions of pounds to strengthen the Union and help stave off a second independence referendum, Theresa May’s Cabinet has been told.

With several political voices warning that the Union is now under threat like never before - including that of David Lidington, the Prime Minister’s de facto deputy – Mrs May told the weekly gathering of her senior ministers – dubbed the “Union Cabinet” – that the “Union had never been more important”.

Several ministers contributed to the hourlong discussion, which heard how support for Scottish independence had increased as Brexit had hit gridlock and that the 2021 Holyrood elections would become “a referendum on a referendum”.

One minister expressed concern that Jeremy Corbyn – who has repeatedly stated that he does not support a second independence referendum – would not oppose a second vote on Scotland’s future if he needed the backing of the SNP to form a government.

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The Cabinet was told that strengthening and sustaining the Union needed to have a “permanent campaign focus” for the UK, requiring resources that ran into tens of millions of pounds.

Ministers were told that far more spending on communications to make the case for the Union was needed as it was pointed out that the UK Government’s overall marketing budget was £440 million but just £100,000 or 0.025 per cent was spent promoting the Union in Scotland.

The discussion on the threat to the United Kingdom and the desire to strengthen it across policy, spending and communications came as last night Tory leadership candidates Boris Johnson and Jeremy Hunt squared up to each other in the one and only live TV head-to-head before a studio audience in Salford.

There is a particular concern among some senior ministers about what a Johnson premiership might mean for the Union in Scotland, especially if there was a no-deal outcome on October 31. Already Nicola Sturgeon has set her aim of holding a second independence referendum “towards the latter half of next year”.

However, Westminster remains the constitutional authority and the PM, as well as the two Conservative contenders to replace her, have made clear they will not facilitate any second referendum; a position the First Minister has branded a “democratic outrage”.

A poll earlier this month showed there was a narrow 51-49 percentage split against independence but when people were asked how they would vote if Mr Johnson became PM, independence gained a six-point lead of 53 per cent to 47.

At Cabinet, another call was made for the UK Government to use more direct funding on Scotland in devolved areas, including a specific fund for promoting culture north of the border. This has been denounced by the Scottish Government as a “power-grab,” which would undermine the devolved settlement.

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It was pointed out to ministers the SNP administration spends £24 million annually on international relations even though this is a reserved matter for Westminster.

A Whitehall source told The Herald: “There was a general acceptance at Cabinet we have to move into campaign mode in terms of how we take forward our commitment to strengthen the Union.

“There was a recognition we must continue to make the case for the Union and it must be properly resourced.”

He added: “The Scottish Government is entirely focused on promoting the case for independence and the feeling is the UK Government has to think in the same way to promote the Union in terms of policy, spending money and communications.”

Last week in her farewell visit as PM to Scotland, Mrs May said the UK must work more "cleverly, creatively and coherently" to maintain it, agreeing with former Labour PM Gordon Brown that it was under serious threat.

Describing Brexit as "a profound constitutional change putting political and administrative strains on the Union", she noted: "When Gordon Brown recently said that he fears the Union is 'more imperilled now than it has ever been' he voiced the fears of many.”

Mr Lidington was even more frank, saying: “The Union is under more pressure than I've known previously in my lifetime.”

The PM has now ordered a review, led by former Scotland Office Minister Lord Dunlop, into how the structures of the UK Government can be best used to strengthen the Union.

The Herald has been told issues being considered include boosting the Scotland Office’s budget and increasing its number of ministers.

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Today, Scottish Secretary David Mundell will appear before MPs to make a staunch defence of his office after some have called for it to be abolished.

Speaking ahead of giving evidence to the Commons Scottish Affairs Committee, he said: “The UK Government will work constantly and whole-heartedly to strengthen the Union and build on the bonds, which hold our Union together. But let me be clear, the devolution settlement is not up for renegotiation; this is not a review of devolution.

“Following the 2016 Scotland Act and the huge transfer of tax and welfare powers it delivered, the present devolution settlement strikes a good balance of powers for Holyrood while Scotland benefits from being part of a strong United Kingdom. We have no plans to make changes to that settlement.”

Mr Mundell added: “The new review will ask the question: ‘Is the UK Government doing all it can to strengthen the Union?’ It is in all our interests that it does.”