Britain must soon take "major steps" towards avoiding a physical border with Ireland after Brexit, European Commission President Jean-Claude Juncker warned on a visit to Dublin on Thursday.
London has vowed no return to frontier checks between British Northern Ireland and EU member Ireland, but has yet to explain how such an arrangement could be compatible with its desire to diverge from the EU's customs and trade arrangements.
"Major steps have to be taken by Britain and I hope that the solutions we have to find will be there," Juncker said following a meeting with Irish Prime Minister Leo Varadkar.
The EU has proposed that Northern Ireland stay aligned with the bloc after Brexit if no other solution to the hard border can be found -- but London says this would break up the UK.
It has suggested instead that the whole country remain aligned with the EU in certain areas, only until the end of 2021, but Brussels knocked that idea back.
Varadkar called for intensified talks if both sides are to reach a deal by their target of October, to leave time for it to be ratified by the British and European parliaments by March 2019, when Britain is due to formally leave the bloc.
"Let me be blunt: there isn't much time left if we are to conclude an agreement and to have it operational by the time the United Kingdom leaves the European Union," the Irish leader said.
Ahead of an EU summit next week, Varadkar highlighted British Prime Minister Theresa May's promise to transcribe the so-called backstop solution on Ireland into legal text.
Referring to London's proposal, he insisted that the backstop "cannot have an expiry date", and urged London to honour its commitments "in full".
"A withdrawal agreement without a backstop is of no use to us whatsoever," he added.
Juncker said Dublin had the support of all other EU member states, warning against "any temptation to isolate Ireland" in the negotiations.