Growing Tory “scorn” could see the “English card” being deployed by Boris Johnson if he becomes prime minister, the former prime minister will say, as polls put support among Scots for independence neck-and-neck with remaining in the UK.

Mr Brown’s intervention comes after Theresa May’s de facto deputy David Lidington warned yesterday that rising English nationalism and a “growing indifference to the Union” south of the Border now stands alongside Scottish nationalism as a threat to the existence of the UK.

First Minister Nicola Sturgeon said Scotland faces its biggest “democratic deficit” in post-war history and demanded that Westminster politicians do not stand in the way of a second independence referendum.

And polling at the weekend found the prospect of frontrunner Mr Johnson becoming prime minister would tip a majority of Scots into backing independence.

In his keynote speech at Westminster, Mr Brown will issue a call to arms from supporters of the Union.

He will say: “The country is about to choose the next prime minister but missing so far from the Conservative leadership contest is a serious debate on the biggest existential question of all – the unity of the United Kingdom and the future of the Union – and how to prevent an even more damaging descent into divisive nationalisms.

“The void in debate is all the more surprising and worrying because the Union is in greater danger than at any point in its 300-year history.

“It is under threat from the SNP’s recent but little-publicised shift from a soft to hard version of independence – with the abandonment of the pound and their desire to leave the UK single market and customs union.

“And it is under threat from the growing scorn for the Union from the Conservative Party membership and from the favourite to be prime  minister Boris Johnson.” Mr Brown will add: “Boris Johnson is on record challenging the very foundations of the modern union - demanding a cut in Scottish representation in the UK parliament, a reduction in the powers of the Scottish Parliament and an end to the Barnett Formula which allocates resources on the basis of need and changing demography.

“We know that the SNP think he is their biggest recruiting sergeant for the independence cause and few think the Union safe in his hands.

“I have no doubt that unless he specifically rules it out the next Conservative Prime Minister will, under the influence of his election guru Lynton Crosby, play the ‘English card’ – whipping up English nationalism against Scotland  for electoral reasons and at a risk to the break up of the Union.”

A recent poll found 63 per cent of Conservative Party members have said they would sacrifice the Union as a price worth paying for Brexit.

The former Labour leader will say: “Their objective is to whip up English nationalist fervour against Scotland simply to secure a Conservative victory – and even at the cost of playing fast and loose with the Union.

“New voices will have to step up and be heard and I for one will fight, fight and fight again for Scotland’s role in Britain.

“I believe the case for Scotland’s role in Britain is strengthened not weakened by recent developments which have demonstrated the scale of our trading links with the rest of the UK, the sheer complexity of our interdependence, the heavy costs of a break up and the real benefits of co-operation.”

The former Chancellor will argue it would be far more difficult to “attempt to disentangle 300 years of integration and interdependence”.

He will say: “It is now urgent that those who support Scotland’s role in the UK put the case against the two divisive extremes, nationalist and Conservative, that threaten to blow the United Kingdom apart.”

Mr Lidington warned during his keynote address in Edinburgh that the rise of English nationalism in recent years poses a growing threat to the UK and politicians face a challenge to keep the sentiment from turning “hostile”.

During the event, staged by the Law Society of Scotland to mark 20 years of devolution, he said defending the Union will be among the most important qualities of the next prime minister and he backed Jeremy Hunt as the best candidate to keep the UK together.

Mr Lidington said: “I do think the union is under more threat than I’ve known previously in my lifetime. I’m a very strong unionist but I think we need to be alive to the fact that there is a combination of nationalist feeling on the one hand and indifference or ignorance of the value of the Union on the other that puts that achievement at risk.”

He added: “The challenge is not just from a strong constituency of support for separatism in Scotland or for pressure from a change in demographic in Northern Ireland, it is in part from an English mood that is sometimes indifferent to the Union and unaware that it is the Union and the efforts of the United Kingdom as a whole that can achieve far more than England would on its own.”

He said he was “shocked” by the polling that showed Conservative Party members would want Brexit delivered, even if it triggers Scotland leaving the UK.

Meanwhile Ms Sturgeon ramped up the pressure for a second Scottish independence referendum, telling the same event that the creation of the Scottish Parliament had been down to the “democratic deficit” which saw successive Tory Governments elected in the 1980s and impose policies north of the Border.

Ms Sturgeon added: “Now 20 years on we face the most important example of a democratic deficit in Scotland’s post-war history.

“The UK Parliament going against the votes of our elected representatives in Parliament and also the votes of the Scottish people in the 2016 referendum - and doing so in a way which pays little or no heed to Scotland’s wishes, priorities and values.”

Ms Sturgeon has set out plans to stage a second referendum on independence next year and legislation has been published at Holyrood which would pave the way to one being held. But it would require the transfer of power from Westminster which has control over the constitution. Both Mr Johnson and Mr Hunt have rejected this.

Ms Sturgeon added: “We need to ask ourselves a fairly fundamental question. “Do we believe that Westminster should have the ultimate right to determine our future regardless of what people in Scotland want?

“Or do we believe, as the Claim of Right, says that people in Scotland have the sovereign right to determine the form of government best suited to our needs.

“My view is that the paramount principle has to be that decisions about Scotland are best taken by the people who live here.

“We can use that principle as our anchor. It determines that Scotland must have the ability if we so choose to chart a different course and choose a different future.”

Gordon Brown says Union is in more danger than ever from nationalism