By Bryony Jones CNN

LONDON (CNN) -- UK Prime Minister Theresa May is to trigger Article 50 Wednesday, kick-starting the Brexit process that will see Britain leave the European Union in two years' time.

The UK voted to withdraw from the EU in a hotly contested and controversial referendum last June, but formal "divorce" proceedings cannot begin until the Prime Minister officially informs Europe that the government is triggering Article 50.

That is expected to happen at 1.30 p.m. Wednesday (7.30 a.m ET) in Brussels, when Tim Barrow, the UK's permanent representative to the EU, delivers a letter of notification to European Council President Donald Tusk. May was photographed signing the letter on Tuesday night.

On Wednesday, at the same time across the channel in London, May will stand to deliver a statement to the House of Commons, confirming that the Brexit process has begun.

Formal notification will start the clock ticking on two years of talks between the UK and the EU to conclude the terms of Britain's exit, and establish future relations between the two parties.

If no deal is reached, the UK will effectively "fall out" of the union on March 29, 2019, two years to the day after Article 50 was triggered.

In her speech to UK Parliament, the Prime Minister will urge Britons divided by the referendum campaign to come together and ensure "we are no longer defined by the vote we cast, but by our determination to make a success of the result," according to extracts released on Tuesday evening.

May will pledge to "represent every person in the whole United Kingdom -- young and old, rich and poor, city, town, country and all the villages and hamlets in between. And yes, those EU nationals who have made this country their home."

And she will say she is determined to create "a truly global Britain that gets out and builds relationships with old friends and new allies around the world."

The bitterly-fought campaign revealed a deep divide across the country, with strong support for the "Remain" campaign in London, Scotland and Northern Ireland, while "Leave" triumphed in Wales and the English regions.

There are renewed fears Brexit could lead to the break-up of the UK.

Scottish lawmakers have called for a fresh independence referendum, since Scotland voted overwhelmingly in favor of remaining part of the EU. But May has indicated she will turn down the referendum request, insisting "now is not the time" for a vote.

Pessimism about the financial implications of Brexit appears to be spreading, too. Just 29% of British households believe leaving the EU will be good for the UK's economy, according to a survey by IHS Markit -- a drop of 10% since July 2016.

The EU is the UK's biggest trading partner, and experts have warned that it may take more than two years to come up with a fresh trade deal.

European Council President Tusk is expected to issue a short statement upon receipt of May's letter, but it is expected he will wait until Friday to make a full response.

The Presidents of the European Parliament and the European Commission, Antonio Tajani and Jean-Claude Juncker and German Chancellor Angela Merkel may address Brexit later Wednesday.

May spoke to Merkel, Tusk and Juncker by phone Tuesday, a day ahead of the official notification.

In separate calls, the four "agreed on the importance of entering into negotiations in a constructive and positive spirit, and of ensuring a smooth and orderly exit process," a Downing Street spokesperson said.

"They agreed that a strong EU was in everyone's interests and that the UK would remain a close and committed ally," the spokesperson added.

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