The United Kingdom formally ended its 44-year cohesion agreement with its continental neighbors on Wednesday when it officially notified Brussels that it was triggering the mechanisms that will withdraw it from the European Union.

A letter signed by Prime Minister Theresa May which invoked Article 50 of the Lisbon Treaty _ the official withdrawal instrument _ was hand-delivered in the early afternoon to European Council president Donald Tusk in Brussels by the UK's ambassador to the EU, Tim Barrow.

Upon receiving the notice, Tusk said on his official Twitter account that, "After nine months the UK has delivered. #Brexit."

In London, May stood up during Prime Minister's Questions in the House of Commons to inform lawmakers in the lower chamber that official notice had been served to Brussels, saying, "this is an historic moment from which there can be no turning back."

She said, however, the UK would remain a close friend and ally of Europe, adding that close relations were in the best interest of both parties.

Tusk responded in a broadcast statement that the split reinforced the remaining 27 members of the bloc to stay united.

"There is nothing to win in this process, and I am talking about both sides," Tusk said. "This is about damage control. Our goal is clear - to minimize the cost for the EU citizens."

May said the UK would use the separation from the EU as a chance to emerge as "stronger, fairer, more outward-looking than ever before."

She did nevertheless acknowledge to the House of Commons that there would be consequences to leaving the EU.

The Conservative leader said that, during the negotiation period in the coming months, she would represent every person in the UK _ young and old, rich and poor from every city, town, village.

She added that she would represent the EU nationals living in the UK, too _ a key issue during the debates leading up to a vote that on Mar. 13 gave the PM parliamentary backing to trigger Article 50.

"These are the ambitions of this Government's Plan for Britain. Ambitions that unite us, so that we are no longer defined by the vote we cast, but by our determination to make a success of the result," said the PM.

"We are one great union of people and nations with a proud history and a bright future," she said of the UK. "And, now that the decision has been made to leave the EU, it is time to come together."

The triggering of Article 50 came just nine months after the British electorate voted 52 percent against 48 to sever ties with the EU in a landmark referendum on June 23, 2016.

The ballot divided opinions in the UK, and several anti-Brexit protesters gathered outside Parliament on Tues. to voice their opposition.

A majority pro-EU Scotland, where 62 percent voters wanted to remain in the bloc, on Tues. passed legislation through its devolved parliament, Holyrood, giving First Minister Nicola Sturgeon official backing to request an independence referendum.

Tusk said that negotiations for Brexit would begin and be "focused on an orderly withdrawal."