Boris Johnson could be the “last Prime Minister of the United Kingdom,” Gordon Brown has warned as he announced the setting up of a new think-tank to make the positive, progressive and patriotic case to maintain the 300-year-old Union.

The former Prime Minister also raised the prospect of an “economic war between Scotland and England” if Britain broke up and he urged Holyrood to be a vehicle for social justice and economic opportunity rather than a “Trojan horse” for Scottish independence.

The stark warning from Mr Brown came as Nick Clegg, the former Liberal Democrat leader, appeared to almost concede defeat on the constitutional question, saying the “Brexit demon” now meant the end of the UK was now “more likely than not”.

As the Conservatives prepare to crown their new leader next Tuesday morning at the QEII Centre, opposite the Commons, the ex-PM told The Herald that the combination of the Conservatives’ failure to put the proper case for the Union coupled with Mr Johnson’s “extreme views” and together with the “SNP’s shift from a soft nationalism to a hard nationalism” with its intention to abandon the pound and leave the UK single market and customs union meant the Union was now under threat like never before.

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It is, he believes, “hanging by a thread”.

Mr Brown argued that the “holding operation” adopted by the Conservatives in which they constantly say no to everything that Nicola Sturgeon demands “may not hold under a Johnson premiership where people see him, whether it’s right or wrong, as having been hostile to Scotland”.

Mr Brown said the danger was that Mr Johnson, if elected next week, could become the last premier of a United Kingdom.

“I want to stop him being the last Prime Minister of the UK. I want to make sure Scotland can have a progressive place within the UK. We have to move on from this battle of the two extremes, this Punch and Judy Show.

“This head-to-head is a zero-sum game that no one can win and I can’t imagine Scotland can do well out of a century of constitutional division and conflict, which is what we are heading for.”

The former PM pointed out how the SNP felt Mr Johnson would be its “best recruiting sergeant”.

“The issue really is whether we can put the progressive case for Scotland’s role in Britain, which Boris Johnson can’t do, and it’s left to people who are not of Mr Johnson’s view but who support Scotland’s role in the UK to put the case now.”

Mr Brown referred to this week’s shocking figures about the record number of drugs deaths in Scotland, saying this illustrated the country’s political problem.

“The minute these figures came out the Conservatives were blaming the Nationalists and the Nationalists were blaming the Conservatives. When lives are being lost as a result of drugs we don’t seem to be able to find a way of working together to solve a problem that is destroying communities. They seem to be more interested in exploiting the divisions than in ending them.

“The Union should be based on co-operation and not on a them-versus-us confrontation. Every time we see a problem the SNP are arguing they need more powers, the Conservatives are arguing they are incompetent…You should be working together to save lives.”

The former party leader said the Conservatives and Nationalists were representing “two extreme positions” but insisted there was a more sensible one.

“Where we feel we are patriots, we are not narrow nationalists, who see everything as a struggle between us and them. We are Scottish but we don’t want to lose our connection to Britain.

“We believe in co-operation as a way forward in an inter-dependent world and not this permanent, endless confrontation and what would end up with a sort of economic war between Scotland and England.”

He went on: “We want to see the Scottish Parliament work as a vehicle for social justice and an economic opportunity and not as some Trojan horse for independence. That’s the position most Scots want to see enacted and that case has to be put.”

Mr Brown stressed: “People want to know what is the positive and progressive case for Scotland both for what the Scottish Parliament can do and what Scotland’s role in Britain is to be and it’s important to recognise why we are talking about a think-tank and doing this seminar – obviously, this has to be built up over a period of time – which is that we think we can make a positive, progressive and patriotic case for Scotland, that doesn’t depend on breaking up the UK.”

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The think-tank, Our Scottish Future, will be headed by Professor Jim Gallagher, the former Whitehall official, who on August 30 will lead its first seminar on which of four currency options – the pound, the euro, sterlingisation or a Scottish pound – would be best for Scotland’s economic future.

Four days earlier at the Edinburgh Book Festival, Mr Brown will give a speech on Scotland, Britain and Europe at an already sold-out event.

Next week, the former PM is also due to give a speech in London covering the issue of a no-deal Brexit.

Elsewhere, Mr Clegg in his first print interview in the New Statesman since leaving politics and becoming Facebook’s head of global affairs and communications in October 2018, said: “It seems to me that the clock is now ticking for the end of the Union of the United Kingdom.

“I am afraid I’ve sort of come to the view that is now more likely than not. The Brexit demon has unleashed such an aggressive and regressive right-wing English nationalism. And that the Conservative Party is converting itself into an English nationalist party.”

Commenting on Mr Johnson, the former Deputy Prime Minister added: “The more you see of him, the less impressive he is. With familiarity, he diminishes.”