EU chiefs Ursula von der Leyen and Charles Michel will sign the post-Brexit trade deal agreed with Britain on Wednesday at 9:30 am (0830 GMT), 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.
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 on Wednesday, this should be enough to head off the threat of a no deal divorce on January 1.
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.