The EU's chief Brexit negotiator says he wants an "all-weather insurance policy" to ensure there's no hard border.
Michel Barnier was speaking after his first meeting with the UK's new Brexit Secretary Dominic Raab.
Both men said time was running out to reach an agreement on the UK's departure from the EU.
Mr Barnier said the Irish border was a key sticking point.
He told reporters that "we have a lot of work to do", adding: "There are 13 weeks left before the October European Council. During this short time we have two things to do.
"We must finalise the withdrawal agreement and we are not yet there.
"We must also prepare a political declaration on our future relationship.
"On the withdrawal agreement, it is a matter of urgency to agree a legally operative backstop for Ireland and Northern Ireland. We need an all-weather insurance policy."
Arriving in Brussels for his meeting with Mr Barnier, Mr Raab told reporters he was looking forward to "heating up the negotiations".
He said progress had already been made, especially in terms of citizens' rights, but it was now "vital" to make progress on the framework of a future relationship between the UK and EU.
Mr Raab said: "I have come out to discuss the detailed proposals we have got in our white paper that you will have seen.
"I'm looking forward to, with renewed energy, vigour and vim, looking at the detail of all of this.
"As Michel (Barnier) has told us, the clock is ticking so I'm looking forward to intensifying, heating up the negotiations and making sure we are in the best position to get the best deal."
Digital desk & PA