The U.K. on Wednesday formally began the process of exiting from the European Union, starting on an unprecedented path to reshape its relationship with its closest allies in some of the most complex negotiations the country has ever undertaken.

Nine months after Britain voted to leave the EU, Tim Barrow, Britain’s ambassador to the bloc, hand delivered a letter to European Council President Donald Tusk notifying the bloc that the U.K. will be the first country ever to leave. U.K. government officials say there is no going back from there.

Mr. Tusk tweeted a photo of himself receiving the notification letter.

“I choose to believe in Britain and that our best days lie ahead,” Prime Minister Theresa May said in Parliament. “We will do all we can to help the European Union prosper and succeed.”

Managing Britain’s exit will be a major political test for Mrs. May, who has only a small majority in Parliament. She faces competing pressure from lawmakers on the right wing of her Conservative party who want a complete cutting of ties with Brussels and from politicians in Scotland, who say they will secede from the U.K. if Mrs. May fails to cut a deal that maintains close trade ties.

Scotland’s Parliament voted on Tuesday to seek a new referendum on independence to be held before the U.K. leaves the EU.

The negotiations will also be a test of the unity of the remaining 27 nations of the EU–which have different priorities–and the endurance of the bloc as a whole; The U.K.’s exit, and the shape of any new deal, will dramatically shift the internal dynamics of the bloc.

The triggering of Article 50 of the Lisbon Treaty, which governs EU law, will open a two-year window for Britain to negotiate the terms of its exit from the bloc, unraveling 44 years of ties.

Many of Britian’s anti-EU politicians have been waiting for this moment for years. “The impossible dream is happening,” said Nigel Farage, one of the leaders of the Brexit movement, in a message from his Twitter account. “Today we pass the point of no return.”

Politicians who supported staying urged Mrs. May to keep close ties with the bloc. “It is crucial that in this two year period the voices and concerns of those who want to preserve close links between Britain and Europe are not shouted down and silenced, and that those with power over this process are held to account,” said Anna Soubry, a Conservative lawmaker.

The negotiations are expected to be tough. Early indications are that Britain and the EU are far apart. Mrs. May has said Britain would pursue a clean break from the EU, regaining control over immigration, leaving the jurisdiction of the bloc’s courts and exiting its common market.

British officials have said they want to negotiate the best new deal they can for trade with the EU. European leaders have said they don’t want to punish the U.K. for leaving, but won’t grant Britain a better deal outside the club than it had in it.

U.K. Treasury chief Philip Hammond said the government is under no illusions that giving up membership of the single market and leaving its customs union could have repercussions for the British economy.

“We understand that we can’t cherry-pick, we can’t have our cake and eat it, and that by deciding to leave the European Union and negotiate a future relationship with the EU as an independent nation there will be certain consequences of that, and we accept that,” he said he said in an interview with the British Broadcasting Corp.

Mrs. May had telephone conversations with Mr. Tusk, German Chancellor Angela Merkel and European Commission President Jean-Claude Juncker on Tuesday, a Downing Street spokeswoman said.

They agreed “that a strong EU was in everyone’s interests and that the U.K. would remain a close and committed ally,” she said. “They also agreed on the importance of entering into negotiations in a constructive and positive spirit, and of ensuring a smooth and orderly exit process.”

Late on Tuesday, Downing Street released a photo of Mrs. May signing the Article 50 letter under a portrait of Robert Walpole, the country’s first prime minister.

One of the first–and likely most contentious–issues in the negotiations is set to be how much the U.K. will have to pay to exit from the bloc, said Iain Begg, a research fellow at the London School of Economics.

While some U.K. officials have said the country shouldn’t have to pay anything, EU officials say the bill, which would include payments for programs the U.K. had committed to before it voted to leave, could be as high as EUR50 billion ($54 billion) to EUR60 billion.

“Failure to settle this thorny, if ultimately straightforward issue quickly could have a damaging effect on other elements of the negotiations,” Mr. Begg said.

Mrs. May wants to negotiate the U.K.’s new trading and financial relationship with the 27 other EU member states, during which she will strive to protect London’s status as a financial hub. But whether she can accomplish that in the tight two-year time span is unclear.

Mrs. May has said one of her priorities is to guarantee EU citizens living in Britain and U.K. citizens living in other parts of Europe continue to have the right to stay even after the U.K. leaves. The EU has said talks on that issue alone will likely take at least several months.

The U.K. and the EU will also have to decide what to do with unspent EU funds that were supposed to go to U.K. farmers, how to implement cross-border security arrangements including access to EU security databases, and whether the U.K. will stay under the jurisdiction of EU regulatory agencies for issues, like nuclear power, that are handled by the bloc’s institutions.

On Thursday, Mrs. May is set to publish details of her plans to transfer EU law into U.K. law, so that 19,000 laws and regulations formed over the past four decades will continue to apply to the U.K. after it leaves the bloc, while allowing Parliament to amend the laws as it deems necessary.

Within 48 hours of Britain triggering Article 50, Mr. Tusk is expected to send draft guidelines to the 27 remaining capitals framing the talks from the EU’s side. Those guidelines should be finally agreed upon at a Brussels summit on April 29.

After that, EU governments and the European Commission, which is set to lead day-to-day negotiations, will likely take several weeks to set a detailed negotiating mandate for Michel Barnier, the EU’s Brexit negotiator. Only then, possibly in late May, are negotiations set to begin in earnest.

Source: Dow Jones