British Prime Minister Theresa May on Wednesday said there could be "no turning back" and called for national unity, after notifying the European Union of Britain's intention to end its 44-year membership of the bloc.
Mr Tusk was himself the first to announce officially that the so-called Article 50 letter had been handed to him in his Brussels office by United Kingdom permanent representative Sir Tim Barrow, revealing the news in a tweet several minutes before Mrs May's statement to the Commons began.
British Prime Minister Theresa May signs the official letter invoking Article 50.
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.
"While we are leaving the institutions of the European Union, we are not leaving Europe", she said.
But there is a third possibility, in which Britain crashes out of the European Union without a deal. But leaving the European Union has threatened the foundations of the peace agreement in Northern Ireland, while Scotland's First Minister Nicola Sturgeon, has pledged to hold another referendum on Scottish independence in late 2018 or early 2019.
"We understand that there will be consequences for the United Kingdom of leaving the EU".
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And he said: 'What can I add to this? "We are sensitive to those concerns, and want you to know that we will share any information on changes that will impact our European footprint as quickly as we can". Formal talks could begin by late spring, but before a deal is enacted, both houses of Parliament will need to vote on it.
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Article 50 of the Lisbon Treaty allows a member state to notify the European Union of its withdrawal and obliges the European Union to try to negotiate a "withdrawal agreement" with that state.
Agreements or any extension to Article 50 will have to be approved by all 27 member states and will likely require ratification by national parliaments. However, the Westminster government has said the deal reached with the European Union will apply equally across all the United Kingdom without exception.
Former U.K. Independence Party leader Nigel Farage, who campaigned for years to take Brexit from fringe cause to reality, said Britain had passed "the point of no return".
"Brexit has made us, the community of 27, more determined and more united than before", he said, assuring that the 27 will stay "united and determined" during the two-year divorce talks.
May added that she believes her country's best days are ahead, and that the British people should be optimistic about the withdrawal.