EU chiefs Ursula von der Leyen and Charles Michel will sign the post-Brexit trade deal agreed with Britain tomorrow morning, officials said.
After that the EU-UK Trade and Cooperation Agreement is expected to be taken to London, where UK Prime Minister Boris Johnson will add his signature.
Ursula von der Leyen is president of the EU Commission while Charles Michel is president of the European Council.
European Commission spokeswoman Dana Spinant tweeted that the signature would be an "important moment".
In a statement, the Commission said the deal would be implemented on only a provisional basis, as there has not been time for the EU parliament to vote on it.
But, with the UK parliament due to ratify the text later tomorrow, this should be enough to head off the threat of a no deal divorce on 1 January.
If Britain had left the EU single market at the end of the year without a trade deal, renewed tariffs and quotes would have damaged cross-Channel trade.
As it is, there will still be a return to a customs and regulatory border after a half-century of close integration, and some level of disruption.
But both sides hope the deal, a hard-fought compromise after ten months of intense negotiations, will form a stable basis to build a new, looser partnership.
It comes as Boris Johnson's post-Brexit trade deal with the European Union won the backing of the Brexiteer backbenchers, who have said the agreement "preserves the UK's sovereignty".
A self-styled "star chamber" of lawyers - led by senior Tory Bill Cash - was assembled by the European Research Group (ERG) to examine the 1,246-page text of the agreement.
They concluded that the agreement "preserves the UK's sovereignty as a matter of law and fully respects the norms of international sovereign-to-sovereign treaties".
In a statement, they said: "The "level playing field" clauses go further than in comparable trade agreements, but their impact on the practical exercise of sovereignty is likely to be limited if addressed by a robust government.
"In any event they do not prevent the UK from changing its laws as it sees fit at a risk of tariff countermeasures, and if those were unacceptable the Agreement could be terminated on 12 months' notice."
The backing of the ERG will be welcomed by Mr Johnson, who could have faced a rebellion on his backbenches without their support.
However, if they had failed to back the deal, it would likely have still passed comfortably tomorrow with Labour support.
European Union Brexit negotiator Michel Barnier said the trade deal struck with the UK was a relief and provided stability for people and companies.
"We've delivered an orderly Brexit," Mr Barnier told France info radio. The last-gasp deal clinched a week before the year-end deadline brought "a little stability," he added.
Mr Barnier said there were still some elements to define in the EU's future relationship with the UK, including on foreign policy co-operation.
Meanwhile the Stormont Assembly in Northern Ireland is set to be recalled to discuss the Brexit trade deal.
It is understood the session will take place tomorrow.
Additional reporting: PA
