The EU's chief Brexit negotiator Michel Barnier has asked UK Brexit Secretary Dominic Raab to provide technical data on how checks and controls along the Irish Sea might work in the event of the backstop coming into play.
Mr Barnier told reporters following several hours of talks with Mr Raab in Brussels that agreeing a legally operational backstop was a matter of "urgency".
"We must have a detailed backstop solution which is legally operational in the Withdrawal Agreement," he said.
"The Prime Minister Theresa May has committed herself to this, as have all the leaders of the EU institutions…
"It's a matter of some urgency. We have to work on drafting an operational backstop, and for that I've asked Dominic and his team to give us a certain amount of data which is necessary for the technical work which we need now on the nature, the place, where and how the necessary controls and checks take place.
"This backstop is critical. It's essential to conclude negotiations, because - as I've said - with no backstop, there can be no agreement."
The Brexit Secretary issued a note of caution, without responding directly to the request for technical work on checks and controls.
Mr Raab told reporters: "On Northern Ireland we remain committed to giving effect to the Joint Report [of last December], continuing the work on the potential solutions, working with Michel and his team on some of the issues he's raised.
"The solutions must be workable. They must be workable for the communities living in Northern Ireland, and living in the Republic of Ireland, for people affected in their daily lives on what Michel and I are negotiating on behalf of the EU and the UK."
The EU and UK agreed a Joint Report last December which provides for a "backstop" to take effect if no other solutions were found to avoid a hard border on the island of Ireland.
The backstop, which would foresee Northern Ireland remain in the single market for goods, and in the customs union, is due to be contained in a protocol to the Withdrawal Agreement, which is due to be concluded in October, or early November at the latest.
The British Government has said that it could not accept a scenario where checks would have to be applied to goods entering Northern Ireland from the rest of the United Kingdom, as that would mean Northern Ireland being treaty differently.
Today's remarks by Mr Barnier indicate that the EU is determined that such a scenario is prepared for.