Britain and the European Union will reconvene post-Brexit trade negotiations in Brussels on Sunday after UK Prime Minister Boris Johnson and EU chief Ursula von der Leyen held inconclusive talks, as time runs out to seal a deal.

The pair spoke for around an hour by phone on Saturday afternoon and agreed to make a final push to finalise an agreement ahead of the Brexit transition period ending on December 31.

The high-level political intervention followed deadlocked UK and EU negotiating teams pausing the last-ditch talks late Friday.

Both sides continue to have "significant differences" on several critical issues that have long stalled progress, a joint statement by Johnson and von der Leyen said.

"Whilst recognising the seriousness of these differences, we agreed that a further effort should be undertaken by our negotiating teams to assess whether they can be resolved," they added.

"We are therefore instructing our chief negotiators to reconvene tomorrow in Brussels. We will speak again on Monday evening."

A deal is seen as essential to avoid deep trade disruption on both sides which would weaken economies already damaged by the coronavirus pandemic.

Johnson and von der Leyen last spoke on November 7, but a month later Britain and the bloc remain divided over so-called level playing field provisions, governance and fisheries.

"Both sides underlined that no agreement is feasible if these issues are not resolved," Saturday's joint statement added.

Britain formally left the EU in January, nearly four years after a referendum on membership that divided the nation, but has remained bound by most of its rules until the end of the year.

Without a new deal, the bulk of cross-Channel trade will revert to World Trade Organization terms, an unwanted return to tariffs and quotas after almost five decades of deepening economic and political integration.

Talks through this year have finalised most aspects of an agreement, with Britain set to leave the EU single market and customs union, bu the most thorny issues have remain unresolved.t

Johnson insists Britain will 'prosper mightily' even without a deal
Johnson insists Britain will 'prosper mightily' even without a deal POOL / Frank Augstein

"We will see if there is a way forward," EU chief negotiator Michel Barnier said Saturday. "Work continues tomorrow."

Britain's top Brexit envoy David Frost will travel to Brussels early Sunday with a small team of negotiators, according to reports, with just days left to finalise an agreement before an EU leaders' summit Thursday.

"This is the final throw of the dice," a government source told UK media.

Johnson has insisted Britain will "prosper mightily" whatever the outcome of the talks, but he will face severe political and economic fallout if he cannot seal a deal.

"If we fail to get an agreement with the European Union, this will be a serious failure of statecraft," Conservative lawmaker Tom Tugendhat, chair of parliament's foreign affairs committee, told the Lowy Institute in an interview.

However, a grouping of eurosceptic Conservatives were urging Johnson to hold firm, telling The Sunday Telegraph "we nothing to fear from freedom".

European capitals have remained remarkably united behind Barnier through the fraught Brexit process, but some internal fractures have now begun to surface.

France on Friday threatened to veto any deal that fails short of their demands on ensuring fair trade and access to UK fishing waters.

Meanwhile a European diplomat told AFP that Belgium, the Netherlands, Spain and Denmark share Paris's concerns that the EU side could give too much ground on rules to maintain competition.

Irish Prime Minister Micheal Martin, whose country could be most affected among EU states by any no-deal scenario, welcomed the continuation of discussions.

"An agreement is in everyone's best interests. Every effort should be made to reach a deal," he tweeted.

German MEP Manfred Weber, the head of the European Parliament's conservative EPP grouping, said it was "now or never" for a deal.

"Boris Johnson needs to make a choice between the ideology of Brexit and the realism of people's daily lives," he said.

"In the middle of the Covid crisis we owe it to our citizens and businesses to find an agreement."