This photo shows the view of the hemicycle inside the European Parliament on the eve of the vote on the Brexit agreement at the European Parliament in Brussels, Belgium. Yves Herman/Reuters

Brexit Day is to be set in stone on Wednesday when the European Parliament in Brussels casts a vote ratifying the terms of Britain's divorce deal from the EU.

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The historic legislative act — the last for the 72 remaining British MEPs who will leave afterwards — will allow Brexit to officially take effect on Friday at 2300 GMT, midnight for most of continental Europe and 11pm in Britain.

European Commission President Ursula von der Leyen and European Parliament President David Sassoli attend the conference of Presidents of the EU Parliament in Brussels, Belgium. Yves Herman/Reuters

British Prime Minister Boris Johnson insists that date will see Brexit "done". But the EU underlines that the next, thornier phase of the split — working out a treaty to define the future relationship between the two sides — is about to begin.

Those negotiations are to go to late this year. At the end of 2020, a transition period easing Britain out of EU rules and regulations will expire, unless extended by both sides -- something Johnson has ruled out.

The transition period, a standstill provision, means ordinary Britons and EU citizens will see little visible change, and indeed London will continue to contribute to the EU budget.

But from this weekend onward, Britain will be excluded from the EU's institutions and decision-making.

Belgian Minister of Foreign Affairs and Defence Philippe Goffin speaks during a press conference in Brussels. AFP

It will officially be what the bloc calls a "third country" -- a non-EU state. A close neighbour, and an important trade and security partner, but on the outside of Europe's single-market club, reduced to 27 nations and a combined population of 450 million.

"Brexit is a loss for us all," European Commission Vice President Maros Sefcovic said after a regular EU ministers' meeting on Tuesday.

He said it was important for the EU "to maintain our unity" as it negotiates the future partnership with the UK.

The British minister for Europe attending the meeting, Christopher Pincher, expressed optimism on what that would yield. "We're looking forward to a very different world and a very different relationship," he said.