That the union is under greater stress than at any time in its 300-year history is something that everyone from Scotland’s first minister to former Conservative and Labour prime ministers and Whitehall thinktanks agree upon.

Nicola Sturgeon told delegates at the SNP conference in Aberdeen on Tuesday that successive Westminster governments had “shattered the case for the union” and that she would demand within weeks the legal powers to hold a second independence referendum in 2020.

The following day the Institute for Government published a report stating that a no-deal Brexit would leave the union at breaking point and that it would be “unsustainable and counterproductive” for the UK government to block another referendum.

John Major, Tony Blair and Gordon Brown have all warned in recent weeks that the government’s handling of Brexit is putting the union in peril. Polling on the eve of the SNP conference showed that support in Scotland for independence had risen to 50%. A higher proportion believed Scotland would be better off economically as an independent country within the EU than if it remained in the UK after Brexit.

“Nothing is inevitable, but this is by far the closest we have come historically to the breakup of the union,” says Scotland’s foremost historian, Tom Devine. “I have always thought that if [the union] was eventually destroyed, the major battering ram would come from south of the border. A particular strain of English nationalism has now infected the Conservative party, and one wonders to what extent the majority in England will be concerned with the loss of Scotland – and indeed Northern Ireland.”

Certainly, there are also concerns that ongoing alignment along the Irish border will inevitably fuel calls for a united Ireland. “There is definitely the sense we are at a pivot point,” says Angus Robertson, the SNP’s former Westminster leader and now managing director of Progress Scotland, an independence polling organisation. “If Boris Johnson and the Tories are saying now that they have a deal that solves all the problems, it doesn’t.”

Play Video 1:53 SNP's Ian Blackford says Johnson's Brexit deal has sold Scotland out – video

In recent polling for Progress Scotland of people open-minded or undecided about independence, 61% agreed that Brexit had made independence more likely and 53% said it had changed their personal views on independence. Robertson says these undecideds make up around 20% of the voting public and would be key to the outcome of any referendum.

“People also say they want to see the impact of Brexit before they decide to do something else,” Robertson adds. “But there has also been a growth in support for another referendum within the next two years, and a sense amongst respondents that there is an inevitability to becoming independent.”

Cat Boyd, a co-founder of the Radical Independence Campaign, a left-green group that carried out voter registration drives on working-class estates in 2014, says it is striking that five years after the first referendum, ideas that were considered fringe – a green new deal, workers on boards, opposing austerity – are now part of the mainstream political discourse in Scotland.

But Boyd worries about the balance of a coming campaign. “The independence movement isn’t just the SNP, as we spent a lot of the 2013-14 campaign telling people, but they will have the majority of the money and resources. I worry that they’ll take the working-class vote – that massive turnout in areas where people never voted before – for granted. If the next campaign is geared towards the professional middle classes, what is there to motivate the rest?”

Devine suggests there should be some focus on what would happen after a vote in favour of independence. The maintenance of a social union with England, as well as what form of trade barrier would exist with Scotland’s biggest market, would depend of the goodwill of the governing party in London at the time.

Furthermore, there would be a pressing need for reconciliation: “The governing party in Scotland has a moral as well as political duty to assuage the doubts of those who will still vote against independence in the next referendum. This is a small country and currently split 50/50. It’s difficult to see with such a profound change how those wounds can be healed in the short-term.”