Theresa May has told EU leaders she will "shortly" table proposals to resolve the vexed issue of the Irish border which is threatening to derail the Brexit negotiations.
Attending the EU Western Balkans summit in the Bulgarian capital Sofia, the Prime Minister reaffirmed her commitment that there should be no return of a "hard border" between Northern Ireland and the Republic.
At the same time she stressed the EU's "backstop" solution - that the North should remain aligned with the EU if there is no Brexit deal - remained unacceptable to the UK.
Her promise to bring forward her alternative plan came as Irish Taoiseach Leo Varadkar warned Britain's EU withdrawal agreement could be in jeopardy unless the deadlock was resolved.
Following what she described as a "very constructive" meeting with Mr Varadkar in the margins of the summit, Mrs May said they were working on what the UK's future customs relationship with the EU would be.
"The commission published a fallback option which was not acceptable to us and we will be bringing forward our own proposal for that fallback option in due course," she said.
Mr Varadkar said he expected the UK to table new proposals within weeks but expressed concern they would not go far enough to resolve the border issue.
"Certainly any move on customs that brings the UK closer to the EU is to be welcomed but resolving the issue of avoiding a hard border requires more than customs," he told the Irish broadcaster RTE.
Earlier, arriving at the Sofia summit, Mr Varadkar said unless there was real and substantial progress" by the time of the next EU summit in June there would be serious questions as to whether there would be a withdrawal agreement at all.
He said it remained an "absolute red line" for Ireland that there must be no hard border on the island and that all 27 remaining EU states supported the current "backstop" proposal.
"If the UK wants to put forward alternatives to that - whether it's an alternative text to the backstop or whether it's some some sort of alternative relationship between the UK and the EU - we are willing to examine that," he said.
"But we need to see it written down in black and white, we need to know that it's workable and legally operable and we have yet to see anything that remotely approaches that."
Earlier Mrs May - who also held talks with European Council president Donald Tusk and commission president Jean-Claude Juncker - denied a report the UK could remain tied to the customs union for years after formally leaving the bloc while the future arrangements are resolved.
"No we are not (climbing down). The United Kingdom will be leaving the customs union, we are leaving the European Union," she said.
Speaking later at a news conference in the Macedonian capital Skopje en route back, she acknowledged there would have to be "arrangements" as to how cross-border matters were dealt with in relation to customs.
However she stressed the UK would be operating its own "independent trade policy" after the end of 2020 when the planned 21-month transition period expires.
Her comments came after Eurosceptics responded with dismay to the suggestion the UK could remain closely aligned to Brussels after 2020.
Jacob Rees-Mogg, leader of the influential European Research Group, told The Daily Telegraph: "We have gone from a clear end point, to an extension, to a proposed further extension with no end point.
"The horizon seems to be unreachable. The bottom of the rainbow seems to be unattainable. People voted to leave, they did not vote for a perpetual purgatory."