LONDON — British and European Union negotiators agreed on Monday on the terms of a 21-month transition period to keep Britain inside Europe’s economic structures and to avoid an economically damaging “cliff edge” departure when the country quits the bloc next March.
The deal, however, depends on a broader agreement on Britain’s withdrawal, due to be finalized this year, which is by no means certain. One of the main sticking points in those talks is the effort to avoid creating a hard border between Northern Ireland, which is part of the United Kingdom, and Ireland, which will remain in the European Union.
The transition accord was announced on Monday at a news conference in Brussels by David Davis, Britain’s main negotiator, and his European Union counterpart, Michel Barnier, who described it as a “decisive step” toward an orderly withdrawal, or Brexit.
To achieve the accord, the British government retreated from an earlier pledge and conceded that European Union citizens who arrived in Britain during the transition period would have the same rights as those already in the country. This year, Prime Minister Theresa May insisted that the rights of those newcomers would be “different.”
Talk in Britain about quitting the European Union’s fisheries policy next year has also evaporated, with Mrs. May accepting that the standstill transition — or “implementation period” as she calls it — meant full compliance with current rules.
Even so, Mr. Barnier made it clear that the transition agreement could not be considered legally binding until after ratification of a wider agreement on withdrawal, which negotiators say they hope to reach in the fall. Among other issues, it would set out how the two sides intend to avoid a hard border in Ireland.
“Nothing is agreed until everything is agreed,” Mr. Barnier said.
European Union leaders are expected to endorse plans for the transition when they meet in Brussels on Thursday and Friday, clearing the way for detailed talks on a future trade relationship.
Even though it is not legally binding, the transition agreement is seen as important for Mrs. May’s government, which has been eager to offer assurances to businesses to help mitigate uncertainty over Brexit. Mr. Davis said that the deal would allow companies “to plan for the future with confidence.”
The Irish border issue is still considered the most complex problem in the negotiations, and it provoked a crisis in December before a fudged, last-minute compromise, was reached.
In a Twitter post, the Irish foreign minister, Simon Coveney, said there would be “no backsliding” on the Irish border issue.
Keir Starmer, the opposition Labour Party’s spokesman on Brexit, called on the government to “prioritize negotiating a final agreement that protects jobs, the economy and guarantees there will be no hard border in Northern Ireland.”
“This agreement could have been signed months ago, but ministers wasted time fighting among themselves, holding out on negotiating objectives that they have failed to achieve and pursuing their reckless red lines,” he added.