Prime Minister Theresa May triggered the two-year divorce process in a six-page letter to EU Council President Donald Tusk, vowing that Britain will maintain a "deep and special partnership" with its neighbors in the bloc. In response, Tusk told Britain: "We already miss you."

May's invocation of Article 50 of the EU's key treaty sets the clock ticking on two years of negotiations until Britain becomes the first major nation to leave the union — as Big Ben bongs midnight on March 29, 2019.

The U.K. joined what was then called the European Economic Community in 1973. Its departure could not come at a worse time for the EU, which has grown from six founding members six decades ago to a largely borderless span of 28 nations and a half billion people. Nationalist and populist parties are on the march across the continent in revolt against the bloc's mission of "ever-closer union." And in Washington, President Donald Trump has derided the EU, NATO and other pillars of Western order built up since World War II.

"This is an historic moment from which there can be no turning back," May told lawmakers in the House of Commons, moments after her letter was hand-delivered to Tusk in Brussels by Britain's ambassador to the EU, Tim Barrow.

(Read also: Brexit and external risk for Indonesia's economy)

In the letter, May said the two sides should "engage with one another constructively and respectfully, in a spirit of sincere cooperation."

But for all the warmth, the next two years will be a tough test of the notion that divorcees can remain good friends.

May is under pressure from her Conservative Party and Britain's largely Euroskeptic press not to concede too much in exchange for a good trade deal with the EU. For their part, the other 27 members of the bloc will need to stick together and stand firm as they ride out the biggest threat in the union's history.

Brexit has been hailed by populists across Europe — including French far-right leader Marine Le Pen — who hope the U.K. is only the first in a series of departures. EU leaders are determined to stop that happening.

"The European Union is a historically unique success story," German Chancellor Angela Merkel said in Berlin. "It remains one even after Britain's withdrawal. We will take care of that."

Britons voted 52 percent to 48 percent in favor of leaving the bloc in a referendum nine months ago, and they remain deeply divided over Brexit.(jun)

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