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.
In a letter to leaders of the EU-27, European Council President Donald Tusk says 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.
The EU issued draft guidelines on Brexit on 31 March.
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:
- On EU citizens living in the UK, he called for “effective, enforceable, non-discriminatory and comprehensive” guarantees
- The UK must fulfil all its financial obligations agreed as an EU member state
- A deal must be reached “to avoid a hard border between the Republic of Ireland and Northern Ireland”
“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”.
- 29 April – EU leaders (excluding the UK) meet in Brussels to adopt Brexit negotiating guidelines
- 8 June – UK parliamentary election – Brexit talks to start soon after the vote
- 24 September – German parliamentary election, with Mrs Merkel seeking a fourth term
- 29 March 2019 – Deadline for ending talks on UK exit terms (any extension requires agreement of all member states)
- May or June 2019 – European Parliament election (without UK)
- Ratification – Any Brexit deal requires ratification by all EU’s national parliaments and European Parliament
Brexit: EU holds summit without UK to formalise strategy – BBC News