The UK leaves the EU in March, and the current plan is for a transition period to finish at the end of 2020.
The UK’s transition out of the EU could be extended by “a matter of months” to ensure there is no visible border in Northern Ireland, Theresa May has said.
The Prime Minister said this was a new idea that had emerged in negotiations and was not expected to be used.
The UK leaves the EU in March, and the current plan is for a transition period to finish at the end of 2020.
Some Tory MPs and Brexit campaigners are angry at the idea of the UK being tied to EU rules for longer.
It comes after a summit of EU leaders in Brussels failed to make decisive progress in reaching an agreement.
Mrs May addressed her 27 European counterparts on Wednesday evening, urging them to give ground and end the current Brexit deadlock.
Speaking on Thursday morning, Mrs May said that the UK had already put forward a proposal to avoid the need for a return to Northern Ireland border posts or a customs border between Northern Ireland and the rest of the UK.
She added: “A further idea that has emerged – and it is an idea at this stage – is to create an option to extend the implementation period for a matter of months – and it would only be for a matter of months.
“But the point is that this is not expected to be used, because we are working to ensure that we have that future relationship in place by the end of December 2020.”
Speaking to media former Conservative minister Nick Boles described extending this period as as “desperate last move” and warned that Mrs May was losing the support of the Tory party.
The Leave Means Leave campaign said a longer transition would give the EU “zero incentive to negotiate anything and gives Brussels the power to force whatever they want on to the UK”.
Irish Prime Minister Leo Varadkar said a longer transition period was not a substitute for a concrete agreement over the backstop.
But he said the idea would have some merit, adding “if it did help to reassure people that the backstop would never be activated, that would be a positive thing”.
Cabinet Office Minister David Lidington said the cost of extending the transition period would have to be “teased out” during the negotiations.
An EU source told the media there would have to be ‘financial implications’ if the UK did extend the transition period.