UK Prime Minister Boris Johnson is coming under pressure to concede more ground to Brussels as hopes for an early breakthrough in the Brexit talks appeared to falter.

UK and EU officials will resume talks in the Belgian capital on Monday with the prospects of an agreement in time for Britain to leave with a deal on October 31 in the balance.

Time is rapidly running out if there is to be an agreement to put to EU leaders to sign off on at their two-day summit starting on Thursday.

The EU's chief negotiator Michel Barnier said "technical-level" talks between officials over the weekend had proved "constructive".

But in a briefing to ambassadors of the remaining EU27 on Sunday in Brussels, he said that "a lot of work remains to be done".

Earlier Johnson told senior ministers that while a "pathway" to a deal could still be seen, there was "still a significant amount of work to get there".

In a Cabinet conference call, he said that they still had to be prepared to leave on Halloween without a deal.

And Leader of the House, Jacob Rees-Mogg, has said the government could achieve a no-deal Brexit by using European law.

"Theresa May got an extension not through UK law, but through EU law and until the 1972 European Communities Act is repealed, EU law is superior law in the UK," he said on BBC's Radio 4 Westminster Hour.

"And the remainiacs all know that, because they know that it takes two to tango and any extension has to be agreed by the council."

The sticking point remains the issue of the Northern Ireland backstop intended to guarantee there is no return of a hard border with the Republic.

Barnier was reported to have raised concern about the complexity of a British plan to keep Northern Ireland in the UK customs territory while avoiding the need for border controls.

There were reported be doubts about the feasibility of the scheme which was said to involve tracking goods as they move through Northern Ireland and then determining the tariff to be paid depending where they end up.

It raised the prospect that negotiations could carry on after this week, with the possibility of an emergency EU summit at the end of the month to finally approve any 11th hour agreement.

However if Johnson cannot get a deal by the weekend, he will come under intense pressure to seek a further Brexit delay, something he has vowed not to do.

Labour however has warned that if necessary it will take action through the courts to force him to comply with the co-called Benn Act which requires to request an extension.

Either way, the stage is set for a major Commons showdown when the prime minister returns to Westminster for an emergency Saturday sitting of Parliament, the first in 37 years.

If he cannot get deal he is widely expected to blame MPs for cutting the ground from under him, laying the ground for a "people versus Parliament" general election.

If he is able to get an agreement, Government sources have said they will seek to rush through legislation to ratify it in time for the promised Halloween withdrawal date.

Some opposition MPs have signalled they could support an agreement if there was a a requirement to put it to the public in a confirmatory referendum.

However Labour leader Jeremy Corbyn indicated he had little enthusiasm for the idea.

"I think many in Parliament, not necessarily Labour MPs but others, might be more inclined to support it (if there was a referendum) even if they don't really agree with the deal. But I would caution them," he said.