Negotiators from the United Kingdom and the European Union got back around the table in Brussels on Monday to seek compromise in a handful of areas standing in the way of a trade deal.

British Foreign Office Minister James Cleverly said on the BBC's Breakfast program the talks would continue "for as long as we have available time, or until we get an agreement", but many assessments were stark.

The talks between the two hugely interdependent trading partners, which have dragged on since the UK left the bloc on Jan 31, could conclude with a free-trade deal that allows them to continue frictionless and tariff-free trading. But they could end in failure, with London and Brussels trading under World Trade Organization terms, a situation business groups say will lead to higher prices and delays.

The Republic of Ireland's leader, Taoiseach Micheal Martin, told Ireland's broadcaster RTE the talks were on a "knife edge".

He said: "My gut instinct is that it is 50-50 right now. I don't think one can be overly optimistic about a resolution emerging."

While the talks have continued, the two sides have traded as if the UK was still a member of the bloc, an arrangement that will end on Dec 31.

With the clock ticking down, experts have varied wildly in their assessment of whether a deal is imminent, or doomed.

The BBC reported on Monday that, after a weekend of tense talks, EU sources said agreement on fishing rights, one of the main areas of contention, was close. Downing Street sources responded by saying it was not. And the EU's chief negotiator, Michel Barnier, agreed with Number 10, telling ambassadors from the EU's 27 member-states on Monday "divergences" remained and that fishing was one of them.

The Guardian newspaper described his assessment as "very gloomy".

British Prime Minister Boris Johnson and European Commission President Ursula von der Leyen vowed to talk on the phone on Monday evening to try to push a deal over the line.

And Cabinet Office Minister Michael Gove went to Brussels for a face-to-face meeting with European Commission Vice-President Maros Sefcovic.

In addition to the deadlock on fishing rights, the two sides have been at loggerheads on how any future deal should be policed, and how a so-called level playing field can be guaranteed for businesses on both sides.

If negotiators do agree a deal, London and Brussels will need to get it written down as a legal text and translated into each member-nation's language before it can be ratified by the European Parliament, the parliaments of the individual 27 member-nations, and the UK Parliament: all before Dec 31.

With "divergences" remaining and many hurdles left to clear, the prospect of chaos at borders on Jan 1 is still very real.