LONDON — Britain will seek a clean break from the European Union, Prime Minister Theresa May said Tuesday in a speech that eliminated any doubt her country would pursue a starkly different path outside the bloc, which for decades has been at the heart of attempts at continental integration.
The speech, long anticipated and rich with detail, was celebrated by Brexit advocates as an endorsement of their most fervent hopes for a full-scale liberation from the dictates of E.U. headquarters in Brussels. E.U. advocates countered that May was steering the country toward a potentially calamitous breakup, leaving Britain with the Donald Trump-led United States as a partner but with few true friends in Europe.
European leaders offered measured responses, suggesting that Britain was becoming more realistic about its prospects in the complex divorce negotiations to come. But they also maintained that the United Kingdom would meet resistance as it seeks to cherry-pick the benefits of the E.U. while throwing off the burdens.
There was no immediate reaction from the incoming American president, who set alarm bells ringing across Europe just a day earlier by signaling he was indifferent to the future of the European Union — and expected more countries to follow Britain’s path out.
Whether that prediction proves accurate could hinge on whether May succeeds or fails in charting a new course — one she said would be independent of E.U. rules on immigration, trade and justice.
After refusing for months to give “a running commentary” on Britain’s negotiating strategy, May offered the clearest indication to date of the country’s departure plans, which were set in motion by last June’s referendum on Britain’s E.U. ties.
May said Britain wants to be “the best friend and neighbor to our European partners” but cannot be “half-in, half-out” of the bloc, which was born from the ashes of World War II and is designed to prevent future conflict by uniting Europe around a common economic and political project.
“We do not seek to hold on to bits of membership as we leave,” she said. She went on to reject preexisting models for quasi-membership that have been favored by those seeking “a soft Brexit.”
Her remarks instead point to a jarring departure that transforms Britain’s relations with Europe.
Britain, she said, will jettison membership in both the single market — which guarantees the free flow of goods, services and people across national boundaries — as well as the customs union, which dictates the terms of trade between Europe and the outside world.
Instead, she said, Britain will seek preferential trade access to European markets through a new agreement. And she said the country would strike out on its own in negotiating trade deals outside the European Union, which will be left with 27 members spanning from Ireland to Cyprus.
Such a break has been widely anticipated, though never formally spelled out. The British pound climbed Tuesday after drops over the previous several days as excerpts of May’s speech began to circulate.
The pound’s value jumped when May said she would give Parliament final say on Britain’s new deal with the European Union. Unlike the country at large, most members of Parliament favored “remain.” May declined to answer when asked what would happen if Parliament nixes the deal.
May’s promise to allow for a transitional period — in which any new agreement is phased in — also seemed to please investors. British businesses have been concerned about the potential for a disruptive “cliff edge” in which the impact of an exit kicks in overnight.
Britain voted by 52 to 48 percent in June to leave behind the European Union after more than four decades of membership in the bloc and its precursors. Britain’s anti-establishment message was seen as prelude to other populist backlashes around the world, led by the election of Trump in November.
May was a reluctant backer of “remain,” but in the months since the vote she has done little to disappoint ardent Brexiteers. She has stressed that British voters want tighter control over immigration, and her words Tuesday suggested that will be her priority in the breakup talks — even at the expense of economic pain from losing membership in the single market and customs union.
Brexit advocates were delighted by May’s plan, while critics despaired.
Former U.K. Independence Party leader Nigel Farage, a Trump ally, tweeted after the speech that May “is now using the phrases and words that I’ve been mocked for using for years. Real progress.”
But Tim Farron, leader of the pro-E.U. Liberal Democrats, told the BBC that May was careening toward a destructive Brexit that would harm the country’s self-interest. “This is a theft of democracy, a presumption that the 51.9 percent of people who voted to leave meant the most extreme version of Brexit available,” he said.
Farron’s ally, former deputy prime minister Nick Clegg, said May was effectively “siding with Donald Trump and against [German Chancellor] Angela Merkel” and thereby “virtually guaranteeing that acrimony rather than compromise will prevail.”
May’s speech came a day after the publication of an interview in Britain’s Times of London and Germany’s Bild in which Trump dismissed the European Union as “a vehicle for Germany” and said that Britain was “so smart in getting out.”
Trump, who backed Brexit, also expressed enthusiasm for a free-trade deal between the United States and Britain. Such a deal would be possible only if Britain leaves the customs union.
May welcomed those remarks, saying Tuesday that “Britain is not at the back of the queue for a trade deal with the United States, the world’s biggest economy, but front of the line.”
The comment referenced President Obama’s intervention in British politics last spring, when he urged Britons to say no to Brexit and insisted that the U.K. would have to wait its turn before negotiating an agreement with the United States should it leave the E.U.
May, who has been in office since July, has repeatedly promised to trigger the start of Britain’s exit talks by the end of March. Once that’s done — through a mechanism known as Article 50 of the E.U.’s Lisbon Treaty — Britain will have two years to negotiate the terms of its departure.
Europe has signaled it will take a hard line with Britain. At a time when other E.U. countries are flirting with a departure, allowing Britain to keep the benefits of membership while unshackling itself from the burdens could prompt other nations to speed toward the exits.
May’s Tuesday speech was cautiously welcomed across the English Channel, where leaders had previously derided Britain for wanting to have its cake and eat it, too — a charge that British politicians did not exactly deny.
“Sad process, surrealistic times but at least more realistic announcement on #Brexit,” tweeted European Council President Donald Tusk.
German Foreign Minister Frank-Walter Steinmeier welcomed “a bit more clarity about the British plans. [May] has underlined that Great Britain is seeking a positive and constructive partnership, a friendship, with a strong E.U. That is good.”
The prime minister delivered her speech at a gilded, neoclassical, 19th-century mansion — Lancaster House — in front of an audience that included foreign diplomats. Margaret Thatcher had used the venue 29 years ago to endorse Britain’s single-market membership.
May spoke in front of a white backdrop emblazoned with the words “A Global Britain,” and her speech emphasized the importance of the country’s continuing ties with Europe and beyond.
“We are leaving the European Union, but we are not leaving Europe,” she said.
But amid generally conciliatory words, there were also barbs.
She said any effort by Europe to negotiate “a punitive deal” with Britain would “be an act of calamitous self-harm.”
If Europe fails to negotiate in good faith, she said, she could decide to “change the basis of Britain’s economic model” — words that were interpreted as a thinly veiled threat to turn the U.K. into a tax haven that would undercut E.U. markets.
She also suggested she was prepared to walk away from the negotiating table, an outcome known as “dirty Brexit.”
“No deal for Britain,” she said, “is better than a bad deal for Britain.”
Karla Adam contributed to this report.