British Prime Minister Theresa May has vowed to return to Brussels at the weekend after her meeting late Wednesday with European Commission President Jean-Claude Juncker concluded without a breakthrough on Britain's future relations with the European Union.

"We have had a very good meeting this evening," May said after talks with Juncker in Brussels. "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," the prime minister added, without going into detail on the outstanding issues.

"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 ... in the interests of all our people," May said.

A commission spokesman also said there was "very good progress," and that "work is continuing."

May told parliament before leaving London that a withdrawal agreement with the European Union "has been agreed in principle," and that negotiations were continuing on an outline statement of the future relationship after Brexit.

She promised to present "the whole package" to lawmakers after it has been considered by a special summit of EU leaders on Sunday.

Britain's plan for exiting the bloc is uncertain due to an impasse in parliament over what do with the Irish border, amid concerns that border infrastructure could reignite violence in the former conflict zone.

Downing Street confirmed earlier that May had discussed with her cabinet the possibility of "technological solutions" for keeping an open Irish border after Brexit.

It said the plan, which was previously floated as a way of avoiding a "hard" border with physical infrastructure, would be part of alternative arrangements that could be used instead of the EU's so-called "backstop" to guarantee an open border.

The Brexit deal includes a temporary, last-resort backstop with separate provisions for Northern Ireland from the rest of the United Kingdom.

May insists the backstop is unlikely to be needed because she plans to reach a permanent solution for the border during the 21-month transition period after Britain leaves the EU on March 29.

"If it is necessary to have an interim arrangement ... there are a number of ways in which that can be achieved - the backstop, as identified in the protocol, the extension of the implementation period, or alternative arrangements - and work is being done on them," she said.

May's new focus on technological solutions is seen as an attempt to appease eurosceptics in her Conservative party, who are threatening to vote against the Brexit deal in parliament, partly because they fear it could lead to an indefinite arrangement.

Former Brexit secretary Dominic Raab, who resigned last week to oppose May's deal, said Britain must "stand up to the bullying tactics from Brussels."

Raab urged May to "change course" in her negotiations, accusing her of making three late concessions that "rendered the deal fatally flawed."

Regarding the backstop, he said the EU was "insisting on imposing its regulation on Northern Ireland, without the people of Northern Ireland having any say over those rules."

The second "dangerous concession" was to allow the EU to veto any unilateral ending of the backstop by Britain, Raab wrote in Wednesday's Telegraph newspaper.

He also objected to May's commitment, in the statement on a future relationship, to "build on the single customs territory" envisaged by the backstop.

Northern Ireland Secretary Karen Bradley told lawmakers on Wednesday there was "no way that there will be a situation where we can be bound into the backstop indefinitely."

May insisted on Tuesday that the Brexit deal is "fantastic" for Northern Ireland, despite strong opposition from the territory's Democratic Unionist Party (DUP).