BRUSSELS — For Prime Minister Boris Johnson, one of the prime selling points of his Brexit agreement with the European Union is that Northern Ireland will not be legally severed from the customs territory of Britain.
“It means,” he said on Thursday, “the U.K. leaves whole and entire.”
How long it would stay that way is another matter.
Among the most profound consequences of Mr. Johnson’s proposed Brexit deal, analysts said, is that it could strengthen the centrifugal forces that were already pulling apart the United Kingdom. Scotland’s nationalists said the plan would galvanize them to seek another referendum on Scottish independence, while Irish nationalists quietly welcomed it as one more step toward a reunified Ireland.
Northern Ireland’s Democratic Unionist Party vowed to vote against Mr. Johnson’s plan in Parliament, saying it would drive a “coach and horses” through the Good Friday Agreement. That peace treaty enshrined Britain’s sovereignty over Northern Ireland, unless a majority of people favor uniting with Ireland, and it set up a power-sharing arrangement between unionists and nationalists.
Even before Mr. Johnson’s deal, however, Brexit had exposed the fractures in the United Kingdom. Voters in Scotland and Northern Ireland both opted to stay in the European Union in the 2016 referendum; those in England voted to leave. The years of tortuous negotiations over the terms of Britain’s departure have only deepened the alienation of many in both places.
“It cannot be right that Scotland alone is facing an outcome it did not vote for — that is democratically unacceptable and makes a mockery of claims that the U.K. is in any way a partnership of equals,” Nicola Sturgeon, the first minister of Scotland and leader of the Scottish National Party, said on Thursday.
“It is clearer than ever that the best future for Scotland is one as an equal, independent European nation,” she said. “That is a choice I’m determined to insure is given to the people of Scotland.”
The last time the Scots had that choice — a referendum in 2014 — they voted against leaving Britain by 55 percent to 44 percent. Analysts said the outcome could be reversed in a second vote, given the economic benefits that Scotland is likely to lose by leaving the European Union along with the rest of Britain.
Even in Wales, which voted to leave Europe in 2016, there is evidence of a budding independence movement. While polls rarely show support for it rising above 25 percent, the chaotic politics of Brexit in London have raised doubts among some Welsh.
The situation in Northern Ireland is more complicated. There is less of a push to break away, though a recent poll by Michael Ashcroft, a British pollster and former Conservative Party official, showed that a bare majority of people there would vote to leave the United Kingdom, if given a choice.
That is partly a function of demographics: Catholics, who tend to be nationalist, are growing more rapidly as a percentage of the population than Protestants, who tend to be unionist. But it also reflects tensions over Brexit, particularly since the arrival of Mr. Johnson and his threat to leave the European Union, even without a deal, by the end of October.
Sinn Fein, the Irish nationalist party that once served as the political wing of the Irish Republican Army, said that if Mr. Johnson carried out that threat, Northern Ireland should hold a referendum.
“People from across this society, even those of a British identity, are now seriously questioning whether there are any merits of staying within the Union after Brexit,” Michelle O’Neill, the deputy leader of Sinn Fein, said during a debate at the Labour Party conference in Brighton, England last month.
In that regard, Mr. Johnson’s deal is a mixed blessing. Northern Ireland would remain legally part of the United Kingdom’s customs territory, but it would stay closely aligned with the maze of European rules and regulations.
That would avoid the need for checkpoints on its border with the Irish Republic, but there would still be customs checks between Britain and Northern Ireland. Rather than cutting across the island of Ireland from east to west, the border would run north to south through the Irish Sea.
The goal is to allow nearly seamless trading to continue between the north and Ireland, a member of the European Union. The question is whether even that level of symbolic differentiation, over time, will change the attitudes of the people, shifting their orientation from London to Dublin.
Some of that has already occurred in the two decades since the signing of the Good Friday Agreement, which ended years of sectarian violence and turned checkpoints manned by British soldiers into a distant memory. The economies of north and south are now thoroughly integrated and public agencies serve the whole island.
“We’re accustomed to all-Ireland boards,” said Monica McWilliams, an academic and former politician in Belfast who was involved in the Good Friday negotiations. “Tourism operates as an all-Ireland board. Health operates as an all-Ireland board. There are a lot of precedents for all-Ireland institutions.”
She noted that Mr. Johnson’s agreement would have a symbolic impact, creating a tangible distinction between Northern Ireland and Britain — “a border down the Irish Sea.” That is important in a place where cultural identity and questions of constitutional sovereignty can matter as much as economics.
Still, like most experts, Ms. McWilliams does not predict a referendum on Irish unification for perhaps a decade or more. There is little appetite for one in the Irish Republic, which is more prosperous than Northern Ireland and worries about the cost of absorbing the North’s faltering economy.
Some Irish experts argue that the Democratic Unionists should have embraced Mr. Johnson’s plan. If he had carried out his threat to leave the European Union without any deal — a prospect that now seems less likely — the pressure for Northern Ireland to split from the Union would have been far stronger.
The North would have been isolated and its economy badly damaged, which is the outcome feared by the Scots. Instead, Northern Ireland could now benefit from an arrangement in which both Britain and the European Union have an incentive to make sure its economy stays competitive.
“You only have to look at Scotland’s jealousy toward Northern Ireland,” said Bobby McDonagh, who served as Ireland’s ambassador to London.
Even Sinn Fein officials have reacted warmly to the deal, in part because it does not give the Democratic Unionist Party a veto over staying aligned with the European Union after a few years, as earlier proposals would have.
The Democratic Unionists, Mr. McDonagh noted, were unhappy with drawing any distinctions between Northern Ireland and the rest of the United Kingdom because they viewed it as a first step toward sundering the two.
In that case, he said, the unionists “should have voted against Brexit to begin with.”