The European Union has issued a fresh warning to the UK that a transitional deal after Brexit will not be secured unless the problem of the Irish border is resolved.
European Council president Donald Tusk told MEPs that the UK had created the issue as a result of the Brexit vote and had a duty to find a solution.
His comments came as Theresa May faced a battle in the House of Lords, with peers set to push for the UK to remain in a customs union after leaving the EU - something which they claim could help address the issues around the border between Northern Ireland and the Republic.
The Prime Minister hopes to secure a transitional deal which would see the UK continue to follow EU rules and trade with the bloc on similar terms until the end of 2020, when a comprehensive deal on a new partnership could take over.
Speaking at the European Parliament in Strasbourg, Mr Tusk said he welcomed the progress that had been made in the Brexit talks.
"We want to use the positive momentum in these negotiations to finally settle outstanding issues, such as the solution to avoid a hard border between Ireland and Nothern Ireland," he said.
In a message that the Government could not simply leave it to the Irish and EU to decide what the customs arrangements at the border should be - as some Brexiteers have suggested - Mr Tusk said: "The UK's decision on Brexit has caused the problem and the UK will have to help solve it.
"Without a solution, there will be no withdrawal agreement and no transition.
"Leaders will assess the negotiations in June. In parallel we will start our first talks about the future EU-UK relationship."
Senior officials were meeting in Brussels on Wednesday to hold preliminary discussions about the future relationship for the first time.