Boris Johnson is expected to announce later that the UK has reached an agreement with the EU over post-Brexit trade and security.
Officials in Brussels are believed to be finalising the details of a deal that will come into force when the UK leaves EU trading rules next week.
It follows months of often fraught negotiations between the two sides.
The PM will speak to European Commission President Ursula von der Leyen by phone to sign off the deal.
Mr Johnson is then expected to hold a press conference in Downing Street to announce it.
The announcement had been expected earlier, but it has been delayed while final haggling between the two sides continues.
Irish foreign minister Simon Coveney told RTE Radio there had been “some sort of last-minute hitch” over the small print of the fishing quotas agreement but a deal was still expected later.
A deal would end the prospect of the two sides imposing widespread import taxes – tariffs – on each other’s goods from 1 January, which could have affected prices.
The deal document is thought to be around 2,000 pages long. It is expected that the UK Parliament will be recalled to vote on a law implementing a deal before 31 December, when the Brexit transition period ends.
On the EU side, leaders can decide to apply a deal provisionally before the European Parliament holds a ratification vote early next year. It could also need eventual approval in national EU parliaments, depending on the exact contents of the deal.