A supporter of Scottish independence rides through the town of Kilmarnock in September 2014. (Robert Perry/European Pressphoto Agency)

Scottish leader Nicola Sturgeon defiantly and unexpectedly announced Monday that she will seek a new independence referendum, forcing British leaders to try to prevent their country from ripping apart even as they negotiate a high-wire divorce from the European Union.

With Britain poised to trigger the start of its E.U. departure by month’s end, Sturgeon said she will push for a vote on Scottish independence in autumn 2018 or spring 2019 — probably just before the United Kingdom leaves the union.

The move, Sturgeon said, will allow pro-E.U. Scots to decide whether “to follow the U.K. to a hard Brexit or to become an independent country.”

“It will be Scotland’s choice,” she said in an announcement that caught Britain’s political world, including the government, off guard. “And I trust the people to make that choice.”

But the new plebiscite — to be held no more than 4½ years after Scots voted down an earlier referendum that was supposedly a “once-in-a-generation” decision — instantly deepens Britain’s already considerable identity crisis.

Another shot at independence for Scotland means that the severing of Britain’s decades-old E.U. ties threatens to claim as collateral damage a far more elemental bond: the three-century-old union at the heart of the United Kingdom.

“Brexit now clearly raises questions not only about the future of the U.K., but whether the U.K. has a future,” said Andrew Blick, a constitutional expert at King’s College London.

It will fall to Prime Minister Theresa May to not only negotiate the most complex and consequential change in Britain’s global role in decades but also to persuade Scots to come along for the ride.

In her response to Sturgeon on Monday, May could barely contain her irritation, accusing the Scottish leader of “playing politics with the future of our country.”

“The tunnel vision that the [Scottish National Party] has shown today is deeply regrettable,” May said in a statement to broadcasters. “It sets Scotland on a course for more uncertainty and division.”

May and the U.K. government technically have the power to block another referendum if they choose. But crucially, May did not threaten to do so, perhaps wary of further stoking separatist passions.

A second independence vote raises the possible scenario of British leaders finalizing their E.U. split while also figuring out how to handle their internal breakup with Scotland, which then would probably seek E.U. membership as Europe’s newest nation. Normally, that is a years-long process, and it is not clear whether Scotland would be given an accelerated path.

Scottish independence advocates lost a vote in September 2014 by a margin of 55 percent to 45 percent. At the time, pro-independence leaders suggested there would not be another such chance for 20 or more years.

But Sturgeon, who holds the title of first minister in Scotland, said Monday that Britain’s E.U. exit against Scottish wishes represents a “change in material circumstances” that justifies a second vote far sooner. 

In a referendum in June, voters across the United Kingdom — England, Wales, Scotland and Northern Ireland — favored leaving the European Union by 52 percent to 48 percent. But a substantial majority of Scots — 62 percent — voted to remain. A majority of voters in Northern Ireland also wanted to stay in the European Union.

Sturgeon’s announcement came as speculation mounted that the British prime minister would soon invoke Article 50 — the never-before-used mechanism for leaving the 28-nation bloc — probably by the end of the month.

Sturgeon said she was forced to seek a new referendum after months of negotiations with the British government aimed at softening the impact of Brexit — talks that she said have failed.

“Our efforts at compromise have instead been met with a brick wall of intransigence,” she said.

May has said she is seeking a clean break from the European Union that would leave the country outside both the European single market and its customs union. On Monday, the British Parliament gave its final approval of the Brexit plan.

In her statement Monday, May said she would “negotiate an agreement that is going to work for the whole of the United Kingdom, and that includes the Scottish people.” She also cited polls to argue that “the majority of the Scottish people do not want a second independence referendum.”

May’s fellow Conservatives said they would vote against the bill authorizing a referendum in the Scottish Parliament next week. So did the Labour Party. But with support from the Green Party, Sturgeon’s Scottish National Party appeared to have the votes it needs to push the legislation through.

Polls suggest it is not clear which way Scottish voters would swing if they are given another choice. Surveys show support for independence hovering just at or below 50 percent.

Thomas Lundberg, a political scientist at the University of Glasgow, said Sturgeon may be calculating that the British government will be so preoccupied with negotiating its way out of the European Union that it will not be able to focus on defending its internal union.

“That may be what the Scottish government is thinking: Take advantage of maximum chaos,” Lundberg said.

But by committing to a vote so early, Sturgeon is also gambling. Many analysts had expected her to wait until polls showed a clear majority in favor of independence before pushing for a second vote. The longtime independence advocate has to hope that the final Brexit deal will be toxic enough to Scots that they will take the breakaway path they rejected in 2014. 

Among the factors weighing against her are sharp oil-price drops that have hurt the Scottish economy. Then there is the tricky question of how the border between Scotland and England will function — especially if it becomes a dividing line between one country within the European Union and another outside.

The vote “is a big, big risk,” Lundberg said. “She must realize that if she gets this wrong, it not only will be the end of her political career, it really will be another generation before there’s another chance at independence.”

Karla Adam contributed to this report.