Brussels - British Prime Minister Theresa May has embarked on a tour of European capitals in a last-minute bid to win Brexit concessions, but top EU officials said they could at best offer clarifications to help get the deal through parliament.

Planned vote delayed

May's trip to The Hague, Berlin and Brussels comes on the heels of a decision to delay the planned vote in the British parliament on the Brexit deal May had negotiated with Brussels, amid fears of a crushing defeat.

"We are very much concerned about the postponement of the vote," French European Affairs Minister Nathalie Loiseau said. "We have done a lot help the UK," she added, noting that it was the only deal on the table.

At stake is the withdrawal agreement, which spells out the legal terms of Britain's departure from the EU on March 29, 2019, as well as a joint political declaration outlining goals for the future relationship.

The backstop

May hopes to secure assurances that Britain "cannot be permanently trapped" in backstop provisions aimed at preventing the emergence of a border between the Republic of Ireland and Northern Ireland, said Martin Callanan, a minister of state in her Brexit department.

Eurosceptics are concerned that the backstop, if it came into effect, would bind Britain closely to the EU, without London being able to unilaterally exit the arrangement. The issue is at the heart of many lawmakers' opposition to the deal.

May is seeking "additional legally binding reassurances," Callanan said, indicating that this would not involve reopening the withdrawal agreement.

May has met with Dutch Prime Minister Mark Rutte in The Hague. He later tweeted that they had "a useful dialogue," after having signalled ahead of time that there was little prospect of renegotiation.

No renegotiation

From there she was due to travel to Berlin for talks with German Chancellor Angela Merkel, before heading to Brussels for meetings with European Council President Donald Tusk and European Commission President Jean-Claude Juncker.

Brussels and Berlin have insisted that they will not renegotiate the withdrawal agreement, which was endorsed by EU leaders last month.

Michael Grosse-Broemer, a senior member of Merkel's Conservative bloc, has said that the deal on the table was a fair one, while warning that any concessions would likely lead to significant changes.

However, there are signs that the 27 remaining member states are willing to look at ways of helping May get the deal through parliament.

Tusk had said that the EU "will not renegotiate the deal, including the backstop, but we are ready to discuss how to facilitate UK ratification."

"The deal we have achieved is the best deal possible. It is the only deal possible. There is no room whatsoever for renegotiation," Juncker has added, speaking to EU lawmakers in the French city of Strasbourg.

"But of course there is room, if used intelligently (...) to give further clarification and further interpretations without opening the withdrawal agreement," he said.

"London must do its homework"

Ultimately, however, London must do its homework, said German Secretary of State for European Affairs Michael Roth, ahead of a meeting in Brussels with his EU counterparts.

Meanwhile, Juncker said the backstop was an integral part of Britain's divorce deal, amid fears that a return to border controls on the island of Ireland could reignite decades-old tensions.

"We have a common determination to do everything to be not in a situation one day to use that backstop. But we have to prepare it," Juncker said. "It is necessary for the entire coherence of what we have agreed with Britain, it it is necessary for Ireland."

"Ireland will never be left alone," he added of the EU member.

May's decision to postpone the vote has raised concerns that Britain could crash out of the EU without a deal in place, likely causing mayhem for people and businesses across the bloc. Several member states have said they are ramping up no-deal preparations.