Boris Johnson will stress the importance of the UK and EU reaching a trade deal by the end of the year in his first meeting with the new president of the European Commission.
The PM will hold talks with Ursula von der Leyen in Downing Street.
Once the UK leaves the EU on 31 January, the two sides will begin talks on their future economic relationship.
Mr Johnson has insisted a deal is possible by the end of 2020 and the process will not be extended.
After its 31 January exit, the UK will enter into an 11-month transition period in which it will largely follow EU rules but will not have any representation in the bloc’s institutions. This period will come to an end on 31 December 2020.
The government rejected calls in the Commons from Labour and the Lib Dems for Parliament to be given a vote on extending the transition period, if necessary, this summer to allow more time for negotiations.
Opposition MPs have warned that trade deals typically take years to conclude and, with relatively little time available, the UK risks defaulting to World Trade Organisation rules at the start of 2021, potentially leading to damaging tariffs for some industries.
But Brexit Secretary Stephen Barclay told BBC Breakfast that the UK and EU had agreed in the political declaration to do a trade deal by the end of this year and they are “confident” they will do that.
“Of course there will be planning within Whitehall for any eventuality, but we aim to get a trade deal,” he said.
“Both sides are clear they want a trade deal, the political declaration says it can be done to that timetable, let’s be positive.”
And No 10 said Mr Johnson was expected to tell the EU president that he is confident of getting a deal and, having waited for more than three years to leave the EU, both British and EU citizens expect the next phase of trade negotiations to conclude on time.
Downing Street added that the PM was likely to underline to Mrs von der Leyen and the EU’s chief negotiator Michel Barnier that the objective was securing an ambitious, tariff-free trade agreement rather than continued regulatory alignment.
Mrs von der Leyen, a former German defence minister, took over from Jean-Claude Juncker at the start of December. She will set out her vision for future UK-EU relations in a speech at the London School of Economics – where she was a student in the 1970s.
In public, EU officials have said they will do all they can to reach agreement by the end of the year although, in private, many have cast doubt on whether this is possible without some difficult compromises.
Mrs von der Leyen has previously warned the UK that the timetable to conclude an agreement is “extremely challenging” and that whatever happened, the EU would remain united and continue to benefit from its single market and customs union.
European Commission spokesman Eric Mamer said the meeting would “set the scene” for the Brexit process, but the European Council needs to approve a mandate in order to begin trade talks “and we are not at that stage yet”.
“This is not a meeting that will go into the details of the trade negotiation per se,” he said.
‘Expect red-line drawing with smiles’
The meeting between Boris Johnson and new European Commission President Ursula von der Leyen is significant in that it’s their first face-to-face in their new roles – but today does not mark start of post-Brexit trade talks.
EU law dictates that trade talks can’t start until the UK legally leaves the bloc. Then EU countries must agree a mandate for the EU Commission to negotiate a comprehensive trade agreement on their behalf.
This mandate then has to be formally signed off at minister level by representatives of all EU countries.
All this means, the EU says, is trade talks will start at the beginning March.
When UK ministers complain that’s too long to wait, the EU response is that the UK always pushed for bigger role for national governments in EU decision-making to make it more democratic.
Expect red-line drawing with smiles today between the prime minister and Mrs von der Leyen – presented as “friends telling each other truths”.
The EU position is that the prime minister’s timetable to get an “ambitious, comprehensive” trade deal agreed and ratified by December is unrealistic.
However, the prime minister will counter this with “truths” of his own, including that negotiations have to be done by December because he won’t extend the transition period.
Legislation implementing the terms of Mr Johnson’s Brexit deal continues to move through the Commons, with the government easily winning all three votes on the Withdrawal Agreement Bill on Tuesday.
The bill will enshrine in law the terms of the transition period, first negotiated by Mr Johnson’s predecessor Theresa May, as well as agreements on citizens’ rights, customs arrangements in Northern Ireland and the UK’s financial settlement.
On Wednesday, MPs will look at parliamentary oversight and issues relating to Northern Ireland.
The DUP, SDLP and Alliance have jointly suggested amendments – suggested changes to the bill – to ensure a legal guarantee that Northern Ireland businesses will have “unfettered access” to the rest of the UK.
The SDLP and Alliance oppose Brexit, while the DUP says the deal would undermine Northern Ireland’s place in the UK.