An Irish border deal must be done by October, Taoiseach Leo Varadkar said.
Otherwise a backstop option will mean Northern Ireland would continue to follow EU regulations after Brexit involving the all-island economy and North-South cooperation, he added.
Unionists have opposed any solution which would create differences between Northern Ireland and the rest of the UK and Prime Minister Theresa May is reliant on Democratic Unionist support in key Westminster votes.
Ahead of a meeting with European counterparts in Brussels Mr Varadkar said: “Everyone takes the view that we will have to have the withdrawal treaty agreed by October because it will have to be ratified by the British parliament and the European parliament, and potentially by some national parliaments. So October is the deadline.
“Would I like to have it done by June? Yes, absolutely. But I would rather have the right deal in October rather than any deal in June.”
He envisaged a really close trading relationship between the UK and the EU, “so close that many of measures in the backstop may become unnecessary”.
The British Government is facing increased pressure to find a solution to the border, one of the most vexed issues of the Brexit negotiations on which there has been little detailed agreement with the EU.
Avoiding a hard border is generally defined as one without frontier checks on goods and services but there has been no evidence of a technological solution for frictionless trade anywhere else in the world, a committee of MPs said.
On Thursday the President of the European Parliament Antonio Tajani said: “The onus is now on the British Government to propose such a solution.
“But I want to make it clear once again that the European Parliament will not give its consent to a withdrawal agreement that does not incorporate solutions to rule out hard borders between the two parts of the island and which can be implemented immediately.
“We must heed the principle that ‘nothing is agreed until everything is agreed’, not least in order to ensure that the discussions on future relations are not hijacked by this delicate question.”
Mr Varadkar reiterated his firm stance on the importance of the backstop arrangement if no other solution is found.
But he said that was not his preferred option and envisaged a “deep” free trade agreement between the UK and EU.
He added: “A customs union partnership between the UK and EU that would be so close to the customs union that it would not necessitate some of the elements that are in the backstop.”
Mr Varadkar’s political opponents in Ireland said letting talks drag on to October put an open border at risk amid hopes a deal could be clinched by June.
Fianna Fail Brexit spokesman Stephen Donnelly said it would be a serious miscalculation, putting Ireland in a weaker negotiating position.
“Ireland’s leverage on the border issue is rapidly declining the further the talks continue between the EU and Britain.
“As the talks progress, many other important issues for other EU member states will come to the fore, with the border becoming one of many competing priorities.”