The EU Withdrawal Agreement could be in question if real and substantial progress is not made in negotiations by June, the Irish premier has said.
Leo Varadkar meets British Prime Minister Theresa May on Thursday in the Bulgarian capital Sofia, at the EU's Western Balkans summit.
The Taoiseach told the Irish Times: "If we are not making real and substantial progress by June then we need to seriously question whether we're going to have a Withdrawal Agreement at all.
"We stand by the text of the Withdrawal Agreement and the text of the protocol published in March as does the taskforce and the 27 member states that are behind us."
Mrs May's Brexit "war cabinet" met again on Tuesday without reaching agreement on which of the two options for customs arrangements on the Irish border - the "customs partnership" and "maximum facilitation" models - it will back.
The EU is putting pressure on Britain to present its preferred option at a meeting of the European Council in June, although Downing Street insists it will not put a timetable on the process.
The Irish border is one of the most vexed issues facing Brexit negotiators.
The UK does not want to see the EU's "backstop" option on the border which would see alignment of Northern Ireland-related matters with the EU, ministers have said.