The future of the United Kingdom is "on a knife edge" as the Tories turn into an English Nationalist Party and begin to write it off, Gordon Brown will argue in a hard-hitting speech.

Speaking at the Borders Book Festival in Melrose tomorrow, the former Labour Prime Minister will deliver a stark message, saying: "I fear that if Scotland has not yet written off Britain, the Conservative Party is starting to write off Scotland. Indeed, if the United Kingdom collapses, it will not be because a majority of Scots are hell-bent on leaving but because the UK Government is giving up on saving it.

"No Union can survive without Unionists and, after an election in which, to head off UKIP, the Conservative and Unionist Party presented itself as the English Nationalist Party, it is clear that the future of the Union is on a knife-edge."

Decrying "London's equivocation over Scotland," the former Kirkcaldy MP will say that Scottish impatience with the institutions of the UK capital has grown so much that the SNP is able, inaccurately, to portray Labour as part of a London-centric elite

No one, he will say should ever automatically equate patriotism with Nationalism or assume that the popular demand for change is more about a change in borders than about the social and economic change people urgently want and have yet to see in their lives.

In his speech, Mr Brown will claim that the SNP and Conservative have spent the last weeks descending into a "sectarian war of words", raising the spectre of Scottish and English vetoes, suggesting that there are irreconcilable differences between the two nations.

He will dismiss as a "dubious constitutional principle" the SNP Government's call for a double majority lock on the EU referendum vote; effectively a Scottish veto should the UK vote as a whole to leave the Brussels bloc after Scotland voted to stay in.

"The irony is," the ex-PM will explain, "that in enunciating such a principle that subordinates the interests of the UK as a whole to the sectional interests of one nation, not just Scotland but also Wales, England and, even more controversially, Northern Ireland, could have a veto that frustrates the rest of the UK's will."

Yet he will note that the Tory Government, instead of exposing the "the illogicality of such vetoes" and emphasising what binds together all four nations of the UK, it is promoting the idea of an English veto.

"It started with English Votes for English Laws(Evel), a policy which would make Scots second-class MPs in the House of Commons, able to vote on only some issues but not all. It continued in the sectionalism of their "English Election Manifesto." It intensified with their poster campaign - the Miliband puppet on strings pulled by Ms Sturgeon or tucked into the pocket of Mr Salmond - which conjured up the idea of the Scottish menace and was designed to whip up English nationalism against Scottish nationalism.

"Even more insidious is the little known Carlisle Principle enunciated during the General Election campaign by Mr Cameron; that each year the UK Government would scrutinise and, in effect, second-guess the work of the Scottish Parliament.

"By focusing on exposing how much Scotland's decisions could harm England, they are now subordinating the interests of the UK as a whole to the sectional interests of their English vote. In a tit-for-tat retaliation to the SNP playing the Scottish card, the Conservatives are playing the English card," Mr Brown will declare.

The former premier will argue that when Scotland's future was at stake in last September's referendum, many patriotic Scots, like himself, chose Unionism over Nationalism. But, he will say, that when the same choice now faces the Tories, they are choosing English Nationalism.

"Sadly, this election tactic - to divide and rule, rather than to unite - is one that the Conservatives can return to again and again, even when they know that by putting party before country an already fragile Union is at even greater risk."

Mr Brown will warn that the SNP will try to claim a divergence of opinion between Scotland and England on welfare cuts and austerity, over the UK's future in Europe and the replacement of the Human Rights Act, more and more people will be tempted to conclude that the Union has become unworkable.

But the ex-PM will insist there is a way through, firstly be embedding a new constitutional principle that recognises the "glue" that binds the UK together: the pooling and sharing of risks and resources to uphold basic social and economic rights from common pensions and free health care to guaranteed help when sick, unemployed or disabled.

"We should state clearly that whatever else any government at Westminster does, it will uphold - and even the most right wing Government will be unable to abandon - the principle of equity between the nations and regions, allocating resources on the basis of need and that it will protect established social and economic rights."

Mr Brown will call for a constitutional convention to set out the rights and responsibilities of each citizen and, if the Government does not agree, that a "People's Convention", made up of opposition parties and groups from civic society, should be established to do so.

"There are already like-minded proposals for a Charter for the Union, and such a convention, which would examine votes at 16, the replacement of the House of Lords by a Senate of the regions and nations, and what could add up to a quasi-federal territorial constitution, would elevate the debate on citizens' rights above the current narrow and blinkered Conservative focus on removing rights rather than affirming them."

In response, the Prime Minister's spokeswoman said David Cameron had been very clear on the Government being a "government for one nation" and respecting all parts of the UK.

She made clear Ministers would bring forward proposals on Evel, that will restrict the voting rights of Scottish MPs, "in due course"; expected early next month.

"What's important here is that this Government is taking forward further devolution in respect of Scotland, Wales and Northern Ireland."

She claimed that the policy of Evel "carefully balances the principle of English consent for English measures with MPs still continuing to deliberate and vote together. This is about respecting the different parts of the UK".

Asked if the PM supported a constitutional convention, the spokeswoman made clear he did not.