The UK's acceptance of a backstop option for the Irish border is clearer than it was in December, the EU's chief Brexit negotiator has said.
The backstop would involve NI, or the UK as a whole, aligning with EU rules required to support North-South cooperation and an all-island economy.
The UK has accepted the need for a backstop to be written into the Brexit withdrawal agreement.
But it has not agreed what EU rules it should cover.
Speaking in Brussels, the EU's chief negotiator, Michel Barnier, said a text for the backstop option would be worked on in the weeks ahead.
The backstop was first agreed to in December, and last month the EU published a draft legal text which would essentially mean Northern Ireland staying in the customs union and the single market for goods.
It was rejected by the UK, with Prime Minister Theresa May saying "no United Kingdom prime minister could ever agree to it".
Mr Barnier said the Irish border question could not be left "pending" and most be brought to an "operational solution".
He said: "This is what Mrs May has formally accepted... there will be a backstop solution when the withdrawal treaty emerges."
He suggested it could still be based on the solution which the prime minister has rejected.
"It may well be the one we have proposed, we have worked on an operational, specific legal basis."
The UK government believes that a backstop will never be needed if its preferred sort of trade deal is negotiated.
On Monday, the prime minister wrote to the European Council President, Donald Tusk, saying "I continue to believe...that we can achieve a close partnership that provides for such a deep trading relationship that specific measures in relation to Northern Ireland are not required".