Theresa May will start a tour of European capitals on Tuesday to ask for improvements to the Brexit agreement after she called off a parliamentary vote on the deal rather than face a crushing defeat.

During a dramatic day at Westminster, the prime minister told a packed Commons chamber that she would seek reassurances from the EU about the Northern Ireland backstop.

She acknowledged that many Conservative MPs feared that the backstop could keep Britain indefinitely within a customs union with the EU. “I have listened. I have heard those concerns, and I will now do everything I possibly can to secure further assurances.”

European Council president Donald Tusk said he would put Brexit on the agenda for Thursday’s meeting of EU leaders, but made clear there could be no renegotiation of the withdrawal agreement or the backstop.

Senior Government sources in Dublin warned against any additions to the current deal such as legal guarantees that would create conditions around the backstop.

One well-placed source said that only a clause to unilaterally withdraw from its provisions would satisfy Mrs May’s critics at Westminster, and that a legal guarantee would be difficult to implement without undermining the backstop. “You can’t do that and preserve the essential quality of the backstop. I don’t see an easy way out.”

British government sources played down the prospect of a breakthrough this week, and the prime minister is unlikely to return to the Commons for a vote on her deal before the middle of January.

Mrs May will travel to the Hague on Tuesday morning for talks with Dutch prime minister Mark Rutte, before flying to Berlin to meet German chancellor Angela Merkel. She is also expected to meet Mr Tusk and European Commission president Jean-Claude Juncker ahead of Thursday’s summit, and to meet Taoiseach Leo Varadkar on the margins of the summit.

Difficult situation

During a phone call with the Taoiseach on Sunday evening, Mrs May outlined the difficult situation she faced, and the fear of many MPs that the provisions of the backstop could apply indefinitely.

It is understood Mrs May raised the prospect of further discussions at an EU level, asking if Mr Varadkar would be open to such a prospect.

Mr Varadkar is understood to have said it would be a possibility, but stressed that the withdrawal agreement could not be reopened.

“I have no difficulty with statements that clarify what is in the withdrawal agreement, but no statement of clarification can contradict what is in the withdrawal agreement,” he said on Monday.

Jacob Rees-Mogg, who leads a group of Conservative backbench Brexiteers, said Monday’s events had brought Mrs May’s downfall closer. “I think it’s been a humiliating day for the country.”

The pound fell after Mrs May announced that she was postponing the vote and at the end of the day was down 1.4 per cent against the dollar at an 18-month low of $1.249, while the euro was up 1.2 per cent at £0.9048.