Mr. Barnier has said that he wants to wrap up talks by October 2018 to enable the European Parliament and member states to assess the results and allow terms for a transition to be agreed on. Although there is legal scope to extend the talks beyond March 2019, when the two-year statutory period expires, such a step would need unanimous approval from the 27 remaining countries.

Ms. Merkel and President François Hollande of France have insisted that London first agree on how to protect the rights of more than about three million expatriates in Britain, and more than one million British citizens living in Continental Europe, and on the amount of money Britain owes to cover its commitments as a member of the bloc.

European officials underlined the need to safeguard the rights of expatriates by referring to them in the opening paragraphs of the nine-page guidelines. The document also said there should be a “single financial settlement” of Britain’s commitments but did not give a figure.

The leaving bill could be roughly 60 billion euros, or $64 billion, according to the European Union authorities, and that sum is already a major sticking point. David Davis, the minister who leads Britain’s Department for Exiting the European Union, told British television on Thursday that he was not expecting a bill “anything like that” size.

Prime Minister Enda Kenny of Ireland has called for steps to preserve a peaceful coexistence with Northern Ireland, where there will be a new land border with the European Union. European officials said “flexible and imaginative solutions will be required” to maintain peace and avoid “a hard border” between Ireland, a European Union member, and Northern Ireland, part of the United Kingdom.