Boris Johnson's chief Europe negotiator David Frost has met his counterpart from the European Union Michel Barnier as talks began to strike a post-Brexit trade deal.
Opening positions set out by the two sides ahead of the negotiations suggested there are major differences on issues including fishing rights, Brussels' demands for a "level playing field" to ensure fair competition and the role of the European Court of Justice.
At the European Commission's Berlaymont headquarters in Brussels, Mr Frost was joined by the UK's ambassador to the EU Sir Tim Barrow and a team of aides.
Talks are expected to continue until Thursday, with a further set of negotiations planned in London later in March.
The UK is refusing to extend talks beyond the end of the year and has warned it will walk away without a trade deal rather than see the process drag on.
A high-level meeting to take stock of progress is scheduled for June, by which time it should be clear whether the Canada-style trade agreement sought by Mr Johnson is possible by the end of the year.
The UK's approach to negotiations states that the "broad outline" of a deal should be apparent by that point, which could then be finalised by September.
If not, "the Government will need to decide whether the UK's attention should move away from negotiations and focus solely on continuing domestic preparations to exit the transition period in an orderly fashion".
Michael Gove, the Cabinet minister responsible for overseeing the UK's exit from the EU, said Britain would not "trade away" its "newly recovered sovereignty" during the talks.
French Europe minister Amelie de Montchalin warned the Prime Minister that the tight December 31 deadline would not pressure the EU 27 into agreeing fresh terms.
Meanwhile, Northern Ireland Agriculture Minister Edwin Poots said neither he nor his Scottish counterpart are considering checks on goods at ports.
Last week, Tanaiste Simon Coveney warned that there would be a "significant impact" on the trade negotiations if the UK does not implement the requirements of the Northern Ireland protocol.
He said "if there isn't progress on the infrastructure needed to implement the Irish protocol as part of the Withdrawal Agreement in the next few months then I think that is going to be a very worrying signal for whether or not it is going to be possible to conclude something sensible before the end of the year".
But Mr Poots yesterday told MLAs: "In the infrastructure that is being sought by Simon Coveney, we are looking at tens of thousands of checks on goods coming from Great Britain to Northern Ireland, which will create a huge pressure on business."
He added: "I should say that neither the Northern Ireland minister nor, indeed, the Scottish minister have expressed that they are willing to accept any checks at any ports.
"Scotland was as firm as I was on that: we would not put infrastructure in our ports to facilitate that. If Mr Coveney had his way, there would be tens of thousands of checks, damaging those just-in-time goods coming from Great Britain to Northern Ireland.
"That would be hugely detrimental to the shops, convenience stores and supermarkets that provide food that has been produced in Great Britain and comes to Northern Ireland. Again, that will have a huge impact on our consumers."