British Prime Minister Theresa May is to hold further Brexit talks in Brussels on Saturday, just hours before a special summit is scheduled to sign off on the European Union withdrawal agreement.

She announced the surprise move after a meeting with European Commission president Jean-Claude Juncker in the city.

Mrs May said: "We have had a very good meeting this evening.

"We have made further progress. And, as a result we have given sufficient direction to our negotiators, I hope, for them to be able to resolve the remaining issues. And that work will start immediately.

"I now plan to return for further meetings, including with President Juncker on Saturday to discuss how we can bring to a conclusion this process and bring it to a conclusion in the interests of all our people."

The remarks came after suggestions from Brussels that the summit set to approve the draft withdrawal agreement on Sunday could be called off unless progress is made on finalising a political declaration on future relations, with one senior official saying: "We're not there yet."

Reports suggested that Germany's ambassador to the EU had said the document must be finalised by tomorrow or Chancellor Angela Merkel would not attend.

Asked whether the UK expected the summit to go ahead, the PM's official spokesman said only: "A summit has been called, an agenda has been published and we look forward to attending."

Earlier, Mrs May and senior British government ministers warned that the UK may not leave the EU if the draft Brexit deal is voted down in the House of Commons.

She told MPs that the alternative to her deal "will either be more uncertainty, more division, or it could risk no Brexit at all".

Mrs May said that if a full deal on the UK's future ties with the EU was not in place by the end of a planned transition period in 2020, there were three options to avoid a hard border in Northern Ireland: a so-called backstop, an extension of the transition or "alternative arrangements".

"We continue to negotiate on that future relationship to get the good deal that we believe is right for the United Kingdom," she said.

The draft deal has angered the DUP and jeopardised the confidence and supply deal which props up Mrs May's minority government.

After a DUP revolt in the House of Commons on Monday, the government accepted a series of amendments yesterday without votes to avoid a repeat.

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Spain has raised concerns about the treatment of Gibraltar in the proposed Brexit text, while France is understood to be pushing for better rights of access to UK fishing waters.

Downing Street said that Mrs May has spoken to Spain's Prime Minister Pedro Sanchez about her commitment to agreeing a deal that works for the whole of the UK, including Gibraltar.

Meanwhile, Mrs Merkel set her face against Conservative Party backbench demands for the UK to have the unilateral power to tear up a proposed "backstop" arrangement for the Irish border.

Mrs Merkel told the German Parliament: "We have placed value, and I think this is right, on the fact that Britain cannot decide unilaterally when it ends the state of the customs union, but that Britain must decide this together with the EU."

Commission vice-president Valdis Dombrovskis said that for Sunday's summit to go ahead, "we will need to have agreed beforehand the political declaration on the future relationship and we are not there yet".