Britain's House of Commons has voted resoundingly to approve a trade deal with the European Union, paving the way to finally completing the UK's years-long Brexit journey.
With just a day to spare before the end-of-the-year deadline, politicians voted 521-73 in favour of the agreement sealed between the UK government and the EU last week.
It will become British law once is passes through the unelected House of Lords later in the day and gets formal royal assent from Queen Elizabeth II.
The UK left the EU almost a year ago, but remained within the bloc's economic embrace during a transition period that ends at midnight Brussels time on Thursday.
European Commission President Ursula von der Leyen and European Council President Charles Michel signed the agreement during a brief ceremony in Brussels on Wednesday morning.
The documents were then being flown to London for British Prime Minister Boris Johnson to add his signature.
The European Parliament also must sign off on the agreement, but is not expected to do so for several weeks.
It has been four-and-a-half years since Britain voted 52 per cent to 48 per cent to leave the bloc it had joined in 1973.
Brexit started on 31 January this year, but the real repercussions of that decision have yet to be felt, since the UK's economic relationship with the EU remained unchanged during the 11-month transition period that ends on 31 December.
The agreement, hammered out after more than nine months of tense negotiations and sealed on Christmas Eve, will ensure Britain and the 27-nation EU can continue to trade in goods without tariffs or quotas.
That should help protect the Stg 660 billion ($A1.2 trillion) in annual trade between the two sides, and the hundreds of thousands of jobs that rely on it.
Mr Johnson said the Brexit deal would turn Britain from "a half-hearted, sometimes obstructive member of the EU" into "a friendly neighbour - the best friend and ally the EU could have".
He said Britain would now "trade and cooperate with our European neighbours on the closest terms of friendship and goodwill, whilst retaining sovereign control of our laws and our national destiny".
The strongly pro-EU Scottish National Party and Liberal Democrats voted against the bill.
But the main opposition Labour Party, which had sought a closer relationship with the bloc, said it would vote for the agreement because even a thin deal was better than a chaotic no-deal rupture.
"We have only one day before the end of the transition period, and it's the only deal that we have," Labour leader Keir Starmer said.
"It's a basis to build on in the years to come."
