(CNN) The European Union has said that resolving the thorny issue of the Irish border must be a priority in Brexit negotiations with the United Kingdom.
European Council President Donald Tusk said Europe should aim to avoid a "hard border" between the Republic of Ireland, which will remain in the EU after Brexit, and Northern Ireland, which leaves as part of the UK.
Border controls between the north and south were eased as part of the Good Friday Agreement, the 1998 accord that brought peace to Northern Ireland after decades of sectarian conflict.
Ireland was expected to ask Saturday that Northern Ireland be allowed to enter the EU if the two Irelands ever unite, an EU Council source confirmed. The EU is not taking a stance on unification, which would be decided by the people of Ireland and Northern Ireland in accordance with the Good Friday Agreement.
Leaders from the 27 EU nations will meet in Brussels, Belgium, on Saturday to adopt the bloc's formal negotiating guidelines.
British Prime Minister Theresa May will not be part of the Brexit summit. Talks between Britain and the European Union are expected to begin soon after the UK snap general election called for June 8.
Setting out negotiating priorities in a letter to 27 EU leaders, Tusk said EU nations would demand progress on "people, money and Ireland" before talking about Britain's future trade relations with the European Union.
The UK government had hoped to be able to negotiate a new trade deal with the European Union at the same time as carrying out the complex process of unraveling a relationship lasting more than 40 years.
But Tusk stressed the need for a "phased approach," in which future relations with Britain will not be discussed until key issues around Britain's continued budgetary commitment to the European Union, the future status of EU citizens living in Britain and the border between Ireland and Northern Ireland are resolved.
"This is not only a matter of tactics, but -- given the limited time frame we have to conclude the talks -- it is the only possible approach," Tusk said. "In other words, before discussing our future, we must first sort out our past," he said.
Tusk said the EU "should aim to avoid a hard border between the Republic of Ireland and Northern Ireland" in order to protect the peace process under the Good Friday Agreement.
A majority in Northern Ireland voted to remain in the European Union in last year's referendum. At present, the "soft" border with the neighboring Republic of Ireland facilitates trade and the movement of people.
Both the UK and Irish governments want to maintain the soft border, but have struggled to define how such an arrangement could work after the introduction of what would be Britain's only land border with the EU.
The issue has forced a discussion on the future status of Northern Ireland: The Good Friday Agreement stipulates that if polls show support for a referendum on unification, then the UK government must offer one. However, a poll by Ipsos MORI in Northern Ireland last September did not suggest great enthusiasm for a united Ireland.
The Irish Prime Minister, or Taoiseach, Enda Kenny called last month for EU leaders to include the language of the Good Friday Agreement in the EU negotiating ground rules.
German Chancellor Angela Merkel on Thursday warned Britain not to hold any "illusions" about what Brexit means for its future, saying that it "cannot and will not have the same rights as a member of the EU."
She also insisted that talks must, from the outset, include Britain's financial obligations to the bloc, the AFP news agency reported.
The size of the so-called Brexit bill is expected to be a key sticking point in negotiations.
Last month, EU Commission President Jean-Claude Juncker told the BBC that Britain would need to stump up roughly £50 billion ($62.4 billion) as it leaves the European Union in order to honor its budgetary commitments.