Brexit: Are Tories worried about the future of the union?
Theresa May's de facto deputy has warned the UK could break up - and indifference in England is partly to blame.
Cabinet Office Minister David Lidington told the BBC the union was under "greater strain" than he had ever seen.
He added that the threat came from "dismissive" views in England, and not just supporters of Scottish independence or Irish unification.
Mr Lidington was speaking ahead of Mrs May's speech on the future of the union in Scotland on Thursday.
He warned a no-deal Brexit would exacerbate tensions - and that if the next prime minister made the wrong decisions, it could mean the end of the UK.
What's going on?
Mrs May is urging her successor as PM to make the union a priority.
Both the men competing to take over from her - Boris Johnson and Jeremy Hunt - have pledged to protect the UK.
But there have been fears in government for some time that a no-deal Brexit could make the break-up of the UK more likely.
- Johnson: Brexit could 'strengthen the union'
- Hunt would not allow UK break-up
- Tory tensions over keeping the UK intact
Mr Lidington is the man responsible for ensuring good relations between the UK government and devolved administrations.
He regularly meets ministers from the Scottish and Welsh governments, and holds talks with key figures in Northern Ireland.
And he's worried.
"The union is under greater strain than I've ever seen in my lifetime," he told me in an interview for BBC Radio 4's World At One .
"I think the UK would be under much greater strain in the event of a no deal," he added.
Asked if the union would be under threat if the new PM made the wrong decisions, he replied: "Yes."
Why does he think this?
There has been tension since the referendum itself.
Scotland and Northern Ireland both voted to remain in the EU, while England and Wales voted to leave.
The Scottish government has said Scots should be asked again about independence as a result, while nationalists in Northern Ireland like Sinn Fein want a border poll on Irish unification.
Mr Lidington fears a no-deal Brexit could increase support for those causes.
What's he said about England?
In short, he argues that too many people are not taking the union seriously enough.
A recent poll suggested around 60% of Tory members would rather Brexit took place even if it meant Northern Ireland or Scotland leaving the UK.
Many Tories I've spoken to have played that poll down.
But Mr Lidington said: "My worry - and it involves the Conservatives but also opinion more widely - is that the threat to the union in my view comes not just from Scottish nationalism and pressure for Irish unification - it comes from indifference among English opinion to the value of the union.
"Sometimes I think there's too many people in England - including in my party - who assume that you can be dismissive about the contribution that Scotland or Northern Ireland makes."
Who is he backing?
Jeremy Hunt. And some Tories think warnings about leaving the UK without a deal are scaremongering an attack on Boris Johnson (although Mr Lidington was reluctant to speak specifically about Mr Johnson during this interview).
The DUP has previously said claims the union would be under threat after a no-deal Brexit are part of "project fear".
But I also spoke to a Johnson backer, Scottish MP Andrew Bowie, who is Theresa May's parliamentary aide.
He argued it would be "mad" not to be worried about the future of the union, but added: "That doesn't mean that you should somehow just accept that the union is going to break up.
"What it means is that we who are unionists have to work harder to make the case for the union."
Mr Bowie added: "It's true that the union is probably facing a greater threat now than it has faced in many, many years.
"But what we need to do is make the case for that union stronger and louder than we ever have before. And it's up to us - elected unionists - to do just that."
What are the candidates saying?
Mr Johnson has made the case that delivering Brexit properly will strengthen the union and make things harder for independence supporters.
For example, he argues it would be hard for the SNP to argue Scotland should rejoin the Common Fisheries Policy, which is unpopular with many in fishing communities.
He has also promised to stress-test policies for the results it may bring to the union if he gets the keys to No 10.
Some of Mr Johnson's backers also fear that if Brexit is not delivered, the Conservatives will lose power to Labour.
They argue that could lead to another referendum in Scotland, if Labour relies on SNP support to stay in power - something Labour dismisses, although some members of the party have suggested it.
Mr Hunt accepts there will be increased demands for Scottish independence in the event of no deal - but says if there were no alternative he would leave without a deal and find a way of protecting the union.
He recently said he thinks the government has been too complacent in making the case for the union.