European Union leaders are meeting in Brussels to discuss a joint strategy for negotiations with the UK over Brexit.
Twenty-seven countries will be present, but the UK will not take part.
The EU will insist that progress must be made in talks on separating the UK from the EU, before any discussions can begin about future trade relations.
Official talks between London and the EU will not begin until after the UK general election on 8 June.
European Council President Donald Tusk spoke about the importance of unity in the remaining 27 countries.
Before entering the talks, he told reporters: “We need to remain united as EU-27. It is only then that we will be able to conclude the negotiations, which means that our unity is also in the UK’s interest.”
In an earlier letter to leaders of the EU-27, he wrote that agreement on “people, money and Ireland” must come before negotiations on the EU’s future relationship with the UK.
The UK government has said it does not want to delay talks on future trade relations.
Mr Tusk’s letter – calling for a “phased” approach to Brexit – echoed German Chancellor Angela Merkel’s priorities, which she set out on Thursday.
“Before discussing our future, we must first sort out our past,” he said, listing three priorities:
“We will not discuss our future relations with the UK until we have achieved sufficient progress on the main issues relating to the UK’s withdrawal from the EU,” he said.
Meanwhile, German Finance Minister Wolfgang Schaeuble said the UK would not have advantages over 27 EU members once Brexit negotiations were concluded.
“There is no free lunch. Britons must know that,” he told Germany’s Funke Media Group.
EU officials estimate that the UK faces a bill of €60bn (£51bn; $65bn) because of EU budget rules. UK politicians have said the government will not pay a sum of that size.
Reports say Irish Prime Minister (Taoiseach) Enda Kenny will also ask his EU partners to back the idea of Northern Ireland automatically joining the EU if the province’s people vote to unite with the Republic.
The UK Brexit Secretary, David Davis, has said that in the event of such a vote, Northern Ireland could become “part of an existing EU member state
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