A letter from Prime Minister Theresa May was sent to EU leaders, formally asking the union to implement the start of exit talks and ending a relationship that had lasted for 44 years.
President of the European Council Donald Tusk confirmed delivery of the letter on social media.
“After nine months the UK has delivered #Brexit”, he tweeted.
"I will not pretend that I am happy for today. But paradoxically there is something positive in Brexit," Tusk added.
"Brexit has made us, the community of 27, more determined and more united than before," he wrote.
Tusk will call a Friday summit with representatives from rest of the EU to agree on political principles to guide the union through the Brexit negotiations.
Speaking in parliament later on Wednesday, May told lawmakers it was “the moment for the country to come together”.
“In accordance with the wishes of the people, the United Kingdom is leaving the European Union,” she said. “This is a historic moment from which there can be no turning back.”
Confirming that clarifying the rights of EU nationals in a post-Brexit Britain would be an early priority, May said she wanted not just to protect the rights of workers, but to build on them.
Once again promising to “represent every person in the whole United Kingdom” during the coming negotiations with the EU, May reiterated her position over a second Scottish independence referendum.
“It is my fierce determination to get the right deal for every single person in this country,” she said.
However, opposition Labour Party leader Jeremy Corbyn urged May to listen more to all parts of the country.
"The next steps along this journey are the most crucial, and if the prime minister is to unite the country, as she says she aims to do, the government needs to listen, consult and represent the whole country, not just hardline Tory ideologues on her own benches," Corbyn said.
Urging unity across the U.K., May added: “As we face the opportunities ahead of us on this momentous journey, our shared values, interests and ambitions can -- and must -- bring us together.”
Britain voted to leave the union in a referendum held on June 23.
Almost 52 percent of registered voters -- mostly in England and Wales -- chose to leave the EU, while 48.1 percent voted to stay.
There are a number of key issues to be negotiated with the other 27 members of the union before the U.K. officially leaves.
These include the timeframe for wrapping up the EU-U.K. talks, which remains unclear but has been an early sticking point ahead of negotiations.
It has been speculated that exit negotiations will continue for around two years. If there is no extension of this deadline, the U.K. will leave the EU on March 29, 2019.
The U.K. is also expected to honor its financial commitments to the EU until its membership formally comes to an end. The total amount London can expect to pay to Brussels until then it estimated at around £52 billion ($65 billion).
Another issue is the future rights of EU citizens living in the U.K. and Britons resident within the EU. This is expected to be a controversial topic for both sides amid widespread uncertainty. Both the U.K. and the EU have said they will seek an early solution.
The U.K.’s borders with the EU, particularly the only land frontier between Northern Ireland and the Republic of Ireland, will need to be urgently addressed.
London and Dublin have both said they do not want to return to a “hard border” in Ireland featuring controls on movement and customs posts but there have been no guarantees.
The border between Spain and Gibraltar -- a British Overseas Territory -- will also be discussed.
The U.K. wants to maintain a free trade agreement with the EU while Brexit talks continue -- a move rejected by the EU. Brussels has said the issue should be dealt with separately.
Access to the single market is also an issue in Scotland, which voted to remain in the EU and wants a special deal on trade with the EU. However, the Westminster government has said the deal reached with the EU will apply equally across all the U.K. without exception.
This week, the Scottish parliament voted to call for a second referendum on independence from the U.K. -- a vote planned to be held in 18 months.
The U.K. applied to join the European Economic Community on Aug. 1, 1961, but did not become a member until Jan. 1, 1973.
A majority of British people voted to stay in the EU in a referendum on June 7, 1975.