Britain's ambassador to the European Union (EU), Tim Barrow, has handed the official letter triggering Article 50 to Donald Tusk, president of the European Council.
This commences the country's two-year exit process from the trading bloc. Tusk's receipt of the letter was confirmed in a tweet sent by him at 13:29 p.m. Brussels time.
British Prime Minister Theresa May addressed the U.K. Parliament shortly afterwards, telling politicians and the live television audience that the process was underway.
"In accordance with the wishes of the British people, the United Kingdom is leaving the European Union. This is an historic moment from which there can be no turning back," affirmed the PM.
"I choose to believe in Britain and that our best days lie ahead," May added, before emphasizing that the U.K. would seek a path of "sincere cooperation" in its dealings with the EU going forward.
"We will continue to be willing allies, close partners and good friends," said the PM, listing several areas her team has identified as particularly lending themselves to future cooperation between the U.K. and the remaining member states.
Yet, Britain's political leader also re-emphasized the primacy of regaining sovereignty and control over immigration as well as not returning to the hard borders of the past in Ireland - all points which are likely to lead to hard-fought discussions between the sides as exact terms are thrashed out.
The hand-delivered letter also drove home the point that the U.K. intends to approach the exit process in a spirit of cooperation despite the British people's vote to leave the EU.
"As I have said before, that decision was no rejection of the values we share as fellow Europeans. Nor was it an attempt to do harm to the European Union or any of the remaining member states. On the contrary, the United Kingdom wants the European Union to succeed and prosper. Instead, the referendum was a vote to restore, as we see it, our national self-determination. We are leaving the European Union, but we are not leaving Europe - and we want to remain committed partners and allies to our friends across the continent," articulated the letter.
Donald Tusk responded on behalf of the EU shortly after PM May's speech concluded in a short announcement tinged with a sentiment of regret and sadness.
"I will not pretend that I am happy today but paradoxically there is something positive in Brexit - it has made the EU 27 more united than before," he stated before commenting that there was nothing to win in the process ahead, rather that "in essence, this is about damage control."
The European Council president added that the remaining member states would "remain determined and united in the difficult negotiations ahead," before finishing his delivery on a poignant note.
"We already miss you," he said.
Although the process is now officially underway, negotiations surrounding the terms under which the U.K. will cease to be a member of the Union are unlikely to kick off before a summit uniting the remaining 27 EU member states scheduled for April 29 in Brussels.
Discussions are anticipated to be challenging with a raft of contentious issues including the free movement of people, trade and the so-called "exit bill" due from the U.K. to the EU.
The extremely thorny question of how to deal with a political border being reinserted between Northern Ireland - which will leave the EU along with the rest of the U.K. - and its Republican sister to the south is also expected to absorb a lot of time and energy.