LONDON — In one of the most consequential diplomatic events in Britain since World War II, Prime Minister Theresa May on Wednesday sent formal notice of the country’s intention to withdraw from the European Union, starting a tortuous two-year divorce littered with pitfalls for both sides.
Speaking in Parliament, May said she was invoking Article 50 of the Lisbon Treaty, putting Britain on track to leave the European Union in 2019 and raising a host of thorny issues involved in untangling a four-decade relationship.
In addition to a welter of trade and customs matters, the Conservative government faces the prospect of a new independence referendum in Scotland, where the majority voted to remain in the European Union, and deep worries about the 1998 Good Friday peace agreement in Northern Ireland.
Just before 12:30 p.m., Britain’s top envoy to the European Union, Tim Barrow, walked to the office of Donald Tusk, president of the European Council, and handed him a letter with the official notification. Tusk then posted on Twitter acknowledging receipt of the letter.
May told Parliament, “Today, the government acted on the democratic will of the British people, and it acts too on the clear and convincing position of this house.”
“The Article 50 process is now underway,” she added, “and, in accordance with the wishes of the British people, the United Kingdom is leaving the European Union.”
She added: “This is an historic moment from which there can be no turning back.” She cited the “enduring power of the British spirit.” And she painted a vision of a “truly global Britain, the best friend and neighbor to our European partners but a country that reaches beyond the borders of Europe too.”
With this step, May enters what William Hague, a former foreign secretary, called “the most complex divorce ever in history.” She begins the process with limited leverage, having made clear that establishing control of immigration takes priority over membership in the European Union’s single market or customs union.
As a result, analysts say, she has frequently stressed her willingness to walk away from the table if a good deal proves elusive, leaving EU negotiators wondering whether she is serious or trying to bluff her way into a stronger negotiating position.
“I think they would prefer a deal,” Anand Menon, a professor of European politics and foreign affairs at King’s College London, said of the British government, noting the high economic stakes. Nevertheless, he said, “I still think they are readier to walk out than most people accept.”
Tusk said at a news conference in Brussels that there was “no reason to pretend this is a happy day,” and that Britain’s plans to depart the bloc would only leave the 27 remaining members “more determined and more united.” He added, “We already miss you.”
Manfred Weber, a German lawmaker and a powerful conservative in the European Parliament, wrote on Twitter that “from now on, only the interests of the remaining 440 million Europeans count for us.” He later added: “If you leave the EU, you lose the associated benefits.”
What makes the looming confrontation so dangerous is that both sides stand to lose economically in the event of a breakdown.
From agriculture to aviation, from fisheries to pharmaceuticals, Britain’s economy has been shaped by membership in the European Union, its main trading partner. With its single market — the world’s largest — and its customs union, the European Union ensures customs-free trade across frontiers in goods and some services.
Without a deal, Britain faces customs checks at its borders and tariffs on imports and exports, not to mention the relocation of at least part of its lucrative financial services sector.
But Britain is a big market for Continental Europe, too: Germany alone exports 800,000 vehicles there every year. Though London’s dominant financial services sector is hardly popular, many businesses on the Continent rely on its deep capital markets.
Britain is one of only two significant military powers in the bloc — the other is France — and has valuable security and intelligence capabilities that elevate Europe on the global stage.
In her letter to Tusk, May argued that without an overall agreement, “our cooperation in the fight against crime and terrorism would be weakened.” Her officials later denied that this was a threat by Britain, arguing that it reflected the fact that some cooperation on policing issues was conducted through EU channels.
But Guy Verhofstadt, the European Parliament’s negotiator for Britain’s exit, said there could be no “trade-off” in talks between such issues and trade. “I think that the security of our citizens is far too important,” Verhofstadt said.
To make matters more difficult, the final deal will require votes in perhaps 38 legislatures across Europe, as well as in the British Parliament.