LONDON — After seemingly endless negotiations, Britain and the European Union on Thursday announced they had struck a post-Brexit trade and security deal, which will reshape relations between the two allies and antagonists for years to come, and may begin to mute the bickering that has consumed the sides in rancorous, often nationalistic debate.

The details of the world’s largest free trade pact were still emerging Thursday, but the deal will allow hundreds of billions of dollars in goods to continue to flow — without tariffs or quotas but a lot more red tape — between Britain and the 27 remaining nations in the E.U., the richest trading club on the planet.

“We’ve taken back control of our laws and our destiny,” British Prime Minister Boris Johnson said at an afternoon news conference from Downing Street. “We’ve taken back control of every jot and tittle of our regulation in a way that is complete and unfettered.”

Johnson said he hoped Europe, too, would be better off with “a prosperous, dynamic and contented U.K. on your doorstep.”

“This deal means a new stability and a new certainty in what has sometimes been a fractious and difficult relationship,” said the British prime minister, adding, “although we have left the E.U., this country will remain culturally, emotionally, historically, strategically, geologically attached to Europe.”

Across the English Channel, there was more nostalgia than celebration.

“Today is a day of relief,” E.U. negotiator Michel Barnier said at a news conference in Brussels. “But tinted by some sadness, as we compare what came before with what lies ahead.”

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European Commission President Ursula von der Leyen said: “It was a long and winding road. But we have got a good deal to show for it. It is fair, it is a balanced deal, and it is the right and responsible thing to do for both sides.”

She added: “Now is the time to turn the page and look to the future. The United Kingdom is a third country. But it is a trusted partner. We share the same values and interests.”

There was palpable relief from business leaders on both sides, who consider this agreement far better than the wreckage of “no deal” exit, something they have feared for years and was previewed this week, when countries shut their borders to travelers from Britain over fears of a coronavirus mutation and thousands of freight trucks were snarled at British ports.

Even with the trade deal, though, the two sides will make a sharp split with little precedent in the modern global economy, with brand-new borders, inspection regimes and paperwork, where for decades there has been none.

Britain officially left the E.U. at the end of January, but little has changed during an 11-month transition period. The real change is coming the night of Dec. 31.

The United Kingdom will leave the E.U. customs union and single market and be able to chart its own course. Johnson envisions a “Global Britain,” a free-trading sovereign nation, able to write its own regulations, control its own borders and make its own deals with the United States and other nations, without seeking consensus in Brussels.

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Many in Britain and abroad, however, fear “Global Britain” will turn out to be runt, a Little England, a diminished power, hobbled by nativism, chasing nostalgic dreams stoked by tabloid newspapers.

The Office for Budget Responsibility, an independent forecaster, estimated that Britain’s gross domestic product will be 4 percent lower in the long-run than if it had stayed inside the bloc. That is greater than the long-term economic impact of the pandemic, the forecaster said.

After Brexit, the E.U. will remain Britain’s biggest trading partner for the foreseeable future, both because of the size of its market and its proximity.

But David Henig, a trade policy expert, said the new relationship will be “nothing like it was” when Britain was a member of the E.U. “It’s always going to look pretty disappointing compared to the current trade situation,” he said.

Food and goods flowing between the two sides will now face a blizzard of paperwork and inspections. Thousands of businesses that have never had to fill out a customs form will now have to do so every time they sell a single item over the border. The U.K. services sector — 80% of its economy — will face steep new barriers, and many British businesses will have to open affiliates on the continent if they want to keep making money in Europe.

For months, Britain and the E.U. struggled to strike a deal over two of the most intractable issues:

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First, how to share the fish swimming in Britain’s coastal waters — the cod, haddock and mackerel that today are mostly caught by fishing trawlers that set to sea under European flags. The deal allows Britain to increase its share to two-thirds of the fishing stock over the next five years.

“We will be able to catch and eat quite prodigious amounts of fish in this country,” Johnson promised.

It’s all about the cod. Boris Johnson threatens a no-deal Brexit as Britain and France fight over fish.

Negotiators also battled over how to assure that the British government would honor its commitments to assure a “level playing field” with the E.U. — meaning that the U.K. would not undercut strict environmental and labor laws, nor grant outsized state subsidies to British businesses, that would give British sectors unfair advantages against its European competitors.

The deal allows each side to appeal to an independent review panel and impose tariffs if they feel wronged.

Barnier, the chief E.U. negotiator, said the deal was limited. It does not address a future partnership on foreign policy, defense and development. Barnier said he was disappointed Britain would no longer participate in a shared university program, known as Erasmus, that has seen a generation of students freely attend colleges on either side of the Channel.

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Johnson said Britain would launch program to send students not just to Europe but around the world.

The trade agreement will still need to be ratified by both sides. The British Parliament will vote on it Wednesday. Some of the arch-Brexiteers in Johnson’s own party signaled that they may oppose the deal. In a joint statement, Mark Francois and David Jones of the European Research Group said their “star chamber” of lawyers would “scrutinise” the deal “to ensure that its provisions genuinely protect the sovereignty of the U.K.” However, Johnson has a comfortable enough majority in Parliament that the deal would be expected to pass.

The deal also appeared to win approval from zealous Brexiteers outside Parliament, with Nigel Farage, leader of the now defunct Brexit Party, tweeting, “The deal is not perfect but it is a big moment … There is no going back.”

On the E.U. side, national capitals will review it. Some parliaments may have to weigh in. The European Parliament will need to sign off, and has said it does not plan to do so before the end of the year because of the deal’s complexity. But the European Union can apply it “provisionally” in the meantime.

German Chancellor Angela Merkel said in a statement that the agreement was of “historic importance.” Berlin would now “intensively examine” the text of the document, she said, adding that she was confident of a “good result.”

French President Emmanuel Macron, who took a tough line in Brexit negotiations, said the deal showed “European unity and firmness have paid off.”

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“The agreement with the United Kingdom is essential to protect our citizens, our fishermen, our producers,” he said. “We will make sure this is the case.”

It often seemed like this day would never come.

Four years ago, Britons stunned the world — and their political establishment — by voting 52-48 to exit the European Union, the first country ever to leave the bloc.

In the months after the lightning-bolt vote in 2016, many E.U. leaders feared other countries could follow Britain out the door. But British leaders have created such difficulties for themselves that few in Europe still worry a departure might tempt anyone else.

Britons tied themselves in knots over how to disentangle from a partnership that has spanned nearly half a century.

The drawn-out melodrama consumed and eventually dashed the premierships of Conservative prime ministers David Cameron and Theresa May, just as John Major and Margaret Thatcher before them were ousted in part over how closely Britain should be tied to the continent.

Johnson rode into 10 Downing Street under the banner “Get Brexit Done.” He and his fellow Brexiteers promised striking a trade deal would be easy. But like everything else with Brexit, it was not. Whereas almost all trade negotiations are about agreeing to give up some sovereignty in exchange for the benefits of a closer relationship, in this one Britain wanted to reclaim sovereignty and diverge.

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“Trade agreements are not made to assert one’s independence. They are made to manage interdependence,” Spanish Foreign Minister Arancha González, herself a former E.U. trade negotiator, told Sky News earlier this month, explaining why the talks had been so difficult. “This trade deal that we are building, post-Brexit, is not to assert people’s sovereignty, countries’ sovereignty. Trade deals are not made to do that.”

The sharp break spelled out by Thursday’s deal was not foreordained. Many Brexit advocates originally envisioned a relationship that would leave their country deeply integrated into the European Union’s market. And at various points, some of those opposed to Brexit hoped Britain might change its mind. Prime Minister May’s circular mantra, “Brexit means Brexit,” let listeners imagine whatever Brexit they wanted.

On the E.U. side, where infighting is common, everyone could agree to agree on Brexit. The comity surprised the British, who appeared to have difficulty understanding why so much deference could be given the views of individual E.U. members, notably Ireland, whose thorny border issues with the U.K. ultimately shaped the broad outlines of the deal.

Rob Ford, a professor of politics at the University of Manchester and author of a new book called ‘Brexitland,” said one of the notable elements of negotiations was the Johnson administration’s willingness to “throw Northern Ireland under the bus, quite openly, really. I mean, Johnson dressed up in his usual rhetorical flimflam, but it ain’t fooled a single unionist voter.”

In five to 10 years, Ford said, politics in Northern Ireland “is going to look pretty different, one way or the other, and historians will look back to this moment and say, well, that’s what did it.”

Some believe Brexit could create conditions that support the break up the United Kingdom. The last general election saw a surge of support for nationalist parties in Northern Ireland and in Scotland, which has its own elections coming up in 2021. In 2016, voters in Northern Ireland and Scotland overwhelmingly voted to stay in the E.U.

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“It’s worth remembering that Brexit is happening against Scotland’s will,” First Minister Nicola Sturgeon said Thursday. “And there is no deal that will ever make up for what Brexit takes away from us. It’s time to chart our own future as an independent, European nation.”

With a deal with Europe in hand, Britain can press the incoming Biden administration for a free-trade agreement, too. Those negotiations will also face challenges. Britons, for instance, have consistently said they don’t want to see America’s hormone-injected beef and chlorine-washed chicken in their grocery stores.

The rupture on Dec. 31 will be felt acutely by those with strong links between the U.K. and the E.U. who find they suddenly need visas, permits, passports.

For decades, millions of Europeans have freely moved between a bloc of countries, living and working where they please. An Italian could, with nothing more than a backpack and a plane ticket, move to London, rent an apartment, get a job, use the National Health Service.

A teenager from Munich could attend university in Manchester and not pay overseas fees. Elderly Brits looking to retire in a sun-kissed country could buy a condo on Spain’s Costa del Sol or a villa in the south of France.

After Brexit, all that will be much more problematic.

Britain will control its borders and has already stated that Europeans will not longer be granted priority visas over others. The new British immigration paradigm will be to recruit “the best and brightest” from India, China or the United States.

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And instead of Romanians and Bulgarians picking 99 percent of the British crops, it will more likely be Nepalese or Bangladeshis on six-month work contracts.

Johnson and his Conservative Party will be working hard to identify and emphasize the benefits of Brexit.

Government ministers even claimed that Brexit enabled Britain to be first in the world to authorize the Pfizer-BioNTech coronavirus vaccine. It wasn’t. Britain remains subject to E.U. medicine rules until the end of the month, and any E.U. country can choose emergency authorization, like Britain did.

But the idea of Britain being able to achieve great success because it is unencumbered by rules and regulations that come with E.U. membership appeals to Brexiteers.

Anand Menon, a professor of European politics at King’s College London, anticipated that there would be more Brexit tub thumping to come. “It’s important for the government to get out a line that ‘we’ve done Brexit and all these good things are coming from it.'”

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Birnbaum reported from Riga, Latvia. The Washington Post’s Quentin Ariès in Brussels contributed to this report.

Video: http://www.washingtonpost.com/video/world/britains-brexit-deal-deadline-is-approaching-heres-whats-at-stake/2020/12/22/239f64f3-658d-474b-bc40-a54f73002711_video.html (REF:arda, REF:tayloraj/The Washington Post)

Video: http://www.washingtonpost.com/video/world/boris-johnson-announces-biggest-trade-deal-in-brexit-agreement-days-before-deadline/2020/12/24/90318e8b-f4b7-4815-a522-94baee00b4c4_video.html (The Washington Post)