European Union leaders approved guidelines Friday for the next phase of Brexit talks with Britain, but set a June deadline for progress on the key issue of Northern Ireland.
At a summit in Brussels, 27 leaders without Britain backed the red lines for talks on the future relationship including trade, and approved a deal for a 21-month transition period.
Prime Minister Theresa May said the move by the remaining leaders created a "new dynamic" for what promises to be tough negotiations ahead of Britain's departure in a year's time.
EU President Donald Tusk said the bloc would now use the "positive momentum" to tackle "outstanding issues", including how to avoid border checks between the British province of Northern Ireland and EU member Ireland.
These would take place "in parallel" with talks on the future trading relationship, he told a summit press conference.
"Leaders will assess in June if the Irish question has been resolved and how to go about a common declaration on our future," he told reporters.
Britain has said it is leaving the EU's single market and customs union, but the prospect of a "hard" border in Ireland has sparked concerns about the fragile peace on the island.
The EU and Britain have agreed a "backstop" that Northern Ireland would remain part of the EU's customs union if there is no better idea - but London is deeply opposed to this.
Mrs May said on Friday: "We will now be sitting down and determining those workable solutions for Northern Ireland but also for our future security partnership and economic partnership."
The adoption of the EU guidelines, and approval of the post-Brexit transition deal, marks a "decisive step in these difficult and extraordinary negotiations", the bloc's Brexit negotiator Michel Barnier said.
But the imposition of a new deadline for progress on the Irish issue steps up the pressure on Britain to find a solution to one of the thorniest problems of Brexit.
The transition deal agreed this week by negotiators effectively maintains Britain's ties with the EU until December 2020, although it will have no voting rights, to allow time for a deal on future relations.
However, the EU guidelines say that "negotiations can only progress as long as all commitments undertaken so far are respected in full".
They call for "intensified efforts" on the outstanding parts of the divorce, adding: "Nothing is agreed until everything is agreed."
Irish premier Leo Varadkar, who met May on the sidelines of the summit on Thursday, said he envisaged a trading relationship that was "so close that many of measures in the backstop may become unnecessary".
Mrs May secured a diplomatic victory on Thursday after securing unanimous EU backing for Britain's assessment that Russia was "highly likely" responsible for a nerve agent attack on an ex-spy in the English city of Salisbury.
"In these difficult circumstances I am especially pleased that despite tough Brexit negotiations the EU had demonstrated unanimous and unequivocal unity with the UK," Mr Tusk added.