British Prime Minister Boris Johnson was to receive Britain's EU divorce agreement Friday after Brussels' top two officials signed it at a closed door ceremony in the dead of night.
Officials from both sides accompanied the document across the Channel to London, where Johnson was to add his signature to those of EU chiefs Ursula von der Leyen and Charles Michel.
Number 10 said that Johnson would sign it later in the day then return it to Brussels, where the EU will make a copy for each party.
On Wednesday next week, the text will go to the European Parliament for ratification and on Thursday diplomats from the EU member states will approve the deal in writing.
Then, on Friday, January 31, Britain spends its last day in the EU before leaving the bloc at 2300 GMT as clocks strike midnight in Brussels.
"Charles Michel and I have just signed the Agreement on the Withdrawal of the UK from the EU, opening the way for its ratification by the European Parliament," European Commission president von der Leyen tweeted.
In a separate tweet, European Council president Michel said: "Things will inevitably change but our friendship will remain. We start a new chapter as partners and allies."
The former Belgian premier, whose council represents EU member governments, added, in French: "I'm keen to write this new page together."