Following the vote in the European Parliament, there will be a farewell ceremony for the British MEPs leaving for the last time.
It was formally signed by the presidents of the European Council, Charles Michel, and the European Commission, Ursula von der Leyen, in Brussels and Mr Johnson in Downing Street.
The agreement settles the terms of Britain’s departure, including future citizens’ rights, the arrangements on the Northern Ireland border and the UK’s divorce settlement.
It also allows for an 11-month transition period, during which the UK will continue to follow EU rules while talks take place on a free trade agreement.
Mr Johnson has said he wants a comprehensive deal – covering all aspects of Britain’s future relationship with the EU, including security – by the end of the year.
He has been adamant that he will not contemplate any extension of the transition period beyond the end of 2020.
However, senior EU figures have repeatedly warned that reaching such a wide-ranging agreement will not be possible within such a tight timetable.
They have cautioned that the UK cannot expect to enjoy the “highest quality access” to European markets if – as Mr Johnson is insisting – it refuses to align with EU rules after Brexit.
With the UK’s instrument to ratify the Withdrawal Agreement due to be deposited with the EU on Wednesday, Foreign Secretary Dominic Raab said it would be a “historic moment”.
“It is the start of a new chapter for an independent, sovereign Britain, looking forward to a decade of renewal and opportunity,” he said.
“Whether we are reducing trade barriers between nations, tackling climate change, or improving lives around the world, our vision of a truly global Britain will be a force for good.”