David Davis, left, Britain’s chief Brexit negotiator, and E.U. chief negotiator Michel Barnier at E.U. headquarters in Brussels on Monday. (Virginia Mayo/AP)

Britain and the European Union began talks Monday to sever their 43-year partnership, kicking off unprecedented divorce negotiations that will shape future relations between them.

Discussions began with an immediate concession from the British over how the talks will be structured, a display of the weakness of the British position in the face of an unusual degree of unity among the E.U.’s 27 remaining members. 

British politics were thrown into turmoil after voters narrowly decided just short of a year ago to leave the E.U., long a source of love-hate angst in British politics. The move toppled one leader and may be close to toppling a second, British Prime Minister Theresa May, after a crippling election earlier this month in which her Conservatives lost their majority.

Despite sharp splits in London over what to seek in the divorce, the lead British negotiator vowed that his nation would plunge onward with a full declaration of independence, dampening expectations after the election that Britain would move to preserve some ties with Brussels.

“Today marks the start of a journey for the United Kingdom and for the European Union,” the British minister charged with negotiating the deal, David Davis, said Monday after a day of meetings with his E.U. negotiating counterpart, Michel Barnier. “There’s no doubt that the road ahead will at times be challenging.”

The Brexit victory shocked even backers of the measure and added momentum to a wave of nationalism and populism in Europe and the United States that was seen as helping elevate Donald Trump to the White House.

But British society has remained deeply divided about the meaning of the Brexit vote and the extent to which leaders should pull out of wide-ranging relationships that have delivered prosperity and frustration to generations of British citizens.

Speaking alongside Davis, Barnier offered a grave outlook about what lies ahead.

“The United Kingdom has asked to leave the European Union. It’s not the other way around,” said Barnier, speaking in French, a decision that itself is a measure of Britain’s waning influence in Europe.

“The consequences are substantial,” he said. But he said that the E.U. approach to Britain will not be “about punishment — it’s not about revenge.”

European leaders have repeatedly said that Britain need not go through with its plans for divorce — although they have been tough about what a split will mean if it happens.

Barnier, a veteran French politician, has been vested by the E.U. to enforce its no-compromise red lines that any deal for Britain must not be more favorable than the one it has as a full member.

His first victory came Monday, when he forced Britain to accept the E.U. timetable for the talks: first a negotiation over the split, and only then a discussion about the future relationship between the two sides. Britain had sought for the talks to proceed in parallel, a structure that would have given London more bargaining power.

The issues at stake are daunting. 

Unresolved is everything from the status of E.U. citizens living in Britain, to intelligence sharing, to the future of tens of thousands of British jobs that could be wiped out if businesses move to Europe to avoid new trade barriers.

So far, European leaders have remained united that Britain cannot have full access to European markets unless it also allows full access to its own. European demands for British restitution have also increased, from $67 billion a few months ago to $112 billion now, a measure of the degree of E.U. toughening against May.

The prime minister is a deeply weakened leader who was badly damaged after parliamentary elections this month swept away her majority. That against-all-odds result means that the British leader is far from assured of staying in her seat, even as the Brexit talks get underway. 

But further turmoil and a new prime minister could risk any progress that is made in the first weeks of talks if the new leader decides to take a different direction.

May plans to present E.U. leaders with a proposal Thursday that would detail British plans for E.U. citizens living in Britain.

Any deal will depend on the willingness of both sides to bargain as the clock ticks toward March 2019, when, under treaty rules, Britain will leave the E.U. whether it has reached a deal about how the new relationship will function or not.

But as Europe grows more confident in its future after the election in France of the pro-E.U. Emmanuel Macron as president and the growing assurance from German Chancellor Angela Merkel that she will be reelected in September, analysts say there may be fewer reasons for Europe to compromise.

 “They can be more relaxed about Britain crashing out without a deal that could destabilize the E.U. economy and destabilize the euro zone,” said Charles Grant, director of the Center for European Reform, a London-based think tank.

Though the basic outlines of a deal could be struck within the allotted time, he said, uncertain British politics could add a challenge.

“The more that Britain is unstable politically, the more difficult it is to complete the talks on time,” Grant said.