IF Theresa May’s Downing Street years were a “bad dream”, to use Boris Johnson’s own words, then for supporters of the Union the next premiership could be their worst nightmare.

Barring an extraordinary upset, a man considered a “liability” for the No campaign in 2014 will tomorrow be named the next Tory leader, taking hold of the keys to Number 10 on Wednesday.

After a campaign in which he refused to answer questions about a police call-out to his home, revealed he didn’t know the details of how the UK would trade with the EU after a no-deal Brexit, and waved a kipper in the air during a hustings, he is – of course – heading for a landslide.

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The Right Honourable Alexander Boris de Pfeffel Johnson, Prime Minister of the United Kingdom of Great Britain and Northern Ireland, First Lord of the Treasury, and leader of the Conservative and Unionist Party.

The saviour of the Union? Thanks for applying, but we’re not sure you’re the best person for the job.

Certainly not according to Gordon Brown anyway, who has warned that Mr Johnson could be the UK’s last prime minister.

After Scots voted in favour of the Union, the fight to preserve the UK had to start all over again after David Cameron delivered his botched victory statement on September 19, 2014.

Theresa May’s attempts to force Brexit through have put more pressure on the bonds that unite the UK, yet her reasonable response to Nicola Sturgeon’s repeated demands for Indyref2 – “now is not the time” – resonated with many in a nation weary of political turmoil.

It demonstrated a clear understanding of the importance of language. She didn’t say “never, ever” – she said, in effect, look at this chaos, do you really think more upheaval is what we need right now? Really?

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Which brings me to the first thing Mr Johnson must get to grips with if he wants to prevent Gordon Brown’s premonition coming true: using the right language.

Earlier in this leadership campaign, Sajid Javid provided a simple lesson in how to get it wrong. “If I become PM, I won’t allow a second Scottish independence referendum,” he said. Allow? Talk about gift-wrapping your words for the SNP.

The only man who could, in theory, prevent a Johnson premiership – Jeremy Hunt – also fell into the same trap. “I will never allow our Union to be broken up.”

Alas, Mr Johnson is not known for choosing his words carefully. This first task for him – delivering a perfectly reasonable “no” to Indyref2 in a perfectly reasonable way – may be beyond his abilities.

If in doubt – ask Ruth Davidson. Her counsel should be used wisely by Mr Johnson, despite their differences.

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Or listen and learn from Gordon Brown’s positive case for the Union.

The former Labour PM has repeatedly warned that “English nationalism” is a grave threat to the UK’s future, and this is something Mr Johnson must urgently address. In his rush to win back voters from Nigel Farage, he cannot allow an indifference to the Union to take hold south of the Border.

There are also a number of practical options the new Prime Minister could consider.

His one-time leadership rival Rory Stewart put forward the idea of a new Department for the Union, with the holder of the Cabinet post acting as the PM’s deputy as First Secretary of State.

The Scotland Office’s role has already increased significantly, and this step would further enhance the UK Government’s commitment to Scotland.

Given Mr Johnson’s propensity for putting his foot in it, appointing a safer pair of hands like Michael Gove to this role could help offset the new PM’s negative ratings among Scottish voters.

It would also allow the office-holder to meet regularly with the First Minister, and help rebuild damaged intergovernmental relations.

When it comes to his own meetings with Nicola Sturgeon, Mr Johnson needs to avoid appearing like a visiting foreign dignitary – something those around him have already recognised, according to one newspaper report last week. Scotland is not a foreign country for the PM, but Mrs May too often gave it that appearance.

He should also seize opportunities to announce new projects north of the Border funded by the UK Treasury, with the wheels already in motion to loosen the purse strings when it comes to Scotland. Mr Johnson needs to reject any demands from disgruntled English Tories to apply the brakes.

A battle for the future of the UK is already under way, whether he likes it or not. When it comes to Scotland’s constitutional future, the Scottish Government is in permanent campaign mode. So why shouldn’t the UK Government have the same mindset? Ministers should consider the impact of every policy decision on the Union – does it give the SNP a fresh grievance and another argument to use for independence? Because, if it does, the Scottish Government won’t hesitate to use it.

Last week’s revelation that Scotland has the highest drug-related death rate in Europe is a case in point. Health is devolved; policing is devolved; justice is devolved; funding for Alcohol and Drug Partnerships has been cut by the Scottish Government… but the UK Government’s stubborn refusal to trial supervised drug consumption facilities in Glasgow handed the SNP and its supporters a way to blame Westminster.

When so many families have been torn apart by tragedy, it was sickening to watch this debate turn into a point-scoring match about the constitution. Both governments must rise above party politics.

Yesterday’s proposal for an emergency summit by the Scottish Government is a better response than it provided last week, and the new Prime Minister must put this crisis near the top of his to-do list.

But there is one looming crisis that Mr Johnson seems determined to be the architect of: a no-deal Brexit. A £30 billion economic hit that could plunge us into recession.

This would not only wreck countless communities across the UK, it could also wreck the Union.

There are lots of things the new Prime Minister can do to help preserve the UK. But there is one thing he should categorically not do – force the country out of the EU without a deal.

If he does, Mr Johnson will risk betraying the very name of the party he leads: the Conservative and Unionist Party.