LONDON (AP) — Britain’s exit from the European Union has been likened to putting toothpaste back in the tube. But it’s more like trying to separate the fluoride from the paste: complicated and messy.

Thursday marks 365 days until Britain officially leaves the EU. The March 29, 2019, departure will end a 46-year marriage that has entwined the economies, legal systems and peoples of Britain and 27 other European countries.

British Prime Minister Theresa May was on a whistle-stop tour of the United Kingdom’s four corners — England, Scotland, Wales and Northern Ireland — to promise a Brexit that unites the country.

“Brexit provides us with opportunities,” May said at a weaving firm in southwest Scotland. “It is in our interests to come together and really seize these opportunities for the future.”

For all her optimism, there are a thousand complex issues to settle, and little time.

Britain formally announced its intention to leave the EU a year ago, triggering a two-year countdown. University of Manchester political science professor Rob Ford said that timeframe is “ludicrously short.”

“That’s not sufficient time to disentangle 40 years of political, social and economic entanglement,” he said. “Even with the best will in the world — which isn’t the spirit in which these negotiations have been conducted — it couldn’t happen.”

Across the English Channel in Brussels, the chief European Parliament Brexit official, Guy Verhofstadt, listed a few of the many areas where the two sides must strike a deal: fishing, aviation, research and academic exchanges, nuclear cooperation and the handling of radioactive materials. Failure could leave British hospitals unable to offer radiation treatment and British planes stranded on the tarmac.

“In every one of these fields it will be necessary to find a new arrangement,” Verhofstadt told The Associated Press. Britain will turn into a third country “and a third country cannot have the same advantages as a member state.”

The EU has repeated that warning ever since Britain voted in June 2016 to leave: Brexit is going to hurt. That applies especially to future trade and economic ties, which the two sides have barely begun to negotiate.

In a speech this month, May said she wanted “the broadest and deepest possible partnership” through a free-trade deal unlike any other in the world. EU leaders warn Britain that it cannot “cherry-pick” the benefits of membership without the obligations.

The two sides have given themselves until October to agree on the outlines of a deal, so that the EU and national parliaments can sign off on it before Brexit day. That deadline is rapidly approaching after many months of delay and deferral.

Nine months passed between Britain voting to leave the EU and the triggering of the two-year countdown. More delay followed when May called a snap election to strengthen her hand in Brexit talks — only to lose her majority in Parliament and much of her authority as leader.

Her government now relies on support from Northern Ireland’s Democratic Unionist Party, which has further complicated talks on the most intractable of all Brexit issues — maintaining the near-invisible border between the EU’s Ireland and the U.K.’s Northern Ireland.

Negotiations between Britain and the EU finally began in earnest last summer. Their main achievement so far is a transition period that will last until the end of 2020. During the transition, Britain will continue to pay into EU coffers and follow the bloc’s rules, though it will lose its voice in decision-making.

The transition period has eased, though not erased, fears of a Brexit cliff-edge, in which time runs out and Britain crashes out of the EU with no deal. Both Britain and the EU — and most businesses — want to avoid that economically and politically destabilizing scenario.

Verhofstadt said he is “an optimist by nature” and considers a cliff-edge Brexit unlikely.

“The question is: Can we bridge the red lines of the U.K. with the principles of the European Union? And the answer is yes, it is possible,” he said.

Amid the uncertainty, British businesses worry. Since the referendum, inflation in Britain has shot up, and growth, once among the highest in the EU, is now below the bloc’s average.

And Britain remains divided.The 52 percent-48 percent referendum result divided Britain into two mutually mistrustful camps, leavers and remainers, battling over the nation’s future.

Remainers argue Britain should be able to change its mind if it turns out Brexit will damage the economy and the country.

“Nobody voted in the referendum to be worse off,” said pro-EU Labour lawmaker Chris Leslie.

That argument infuriates Brexiteers like John Longworth, co-director of lobby group Leave Means Leave. He says pro-EU campaigners are “a fifth column in the U.K. working in collusion with the European Union to try and wreck the Brexit process.”

While Brexit has divided Britain, it has brought out unity in the often fractious EU.

“After Brexit, everybody thought there would be a sort of domino effect,” Verhofstadt said. “A Dexit, the Danish going out; Nexit, the Dutch going out; a Frexit even, the French going out. What we have seen is exactly the opposite. Since Brexit, we see that people again have a positive feeling about the EU.

“They are saying, we will not be so stupid as to leave the EU, to destroy the EU. So Brexit has been a serious wake-up call.”