European Union leaders formally agreed upon a Brexit deal at a Brussels summit on Sunday, urging Britons to back Prime Minister Theresa May‘s package, which faces furious opposition in the British parliament.

After a year and a half of arduous negotiations, the 27 leaders took barely half an hour to rubber-stamp a 600-page treaty setting terms for the UK’s withdrawal from the European Union on March 29 and a 26-page declaration outlining ambitions for a future free trade relationship.

President of the European Council Donald Tusk announced the agreement on Twitter.

“EU27 has endorsed the Withdrawal Agreement and Political Declaration on the future EU-UK relations,” he said.

EU27 has endorsed the Withdrawal Agreement and Political Declaration on the future EU-UK relations. — Charles Michel (@eucopresident) November 25, 2018

EU chief negotiator Michel Barnier said now that the first phase was done, the UK and the EU needed to work for “an ambitious and unprecedented partnership”.

“We will remain allies, partners and friends … now is the time for everybody to take their responsibility – everybody,” he said.

President of the European Commission Jean-Claude Juncker, meanwhile, said the sign-off on the agreement marked a “sad day” but backed May to steer the deal through parliament when it is put to a vote next month.

“I believe that the British government will succeed in securing the backing of the British parliament,” Juncker said, declining to comment on what might happen if May fails.

“I would vote in favour of this deal because this is the best deal possible for Britain,” he added.

Domestic test

May now faces the arduous task of winning parliamentary backing for the deal, which foresees London following many EU rules to keep easy trade access, with the British leader expected to face fierce resistance in the coming weeks from both supporters and opponents of Brexit within her government and other opposition parties.

For those in favour of Britain remaining in the bloc, the agreement is a watered down, inadequate version of the country’s existing membership arrangement with access to its customs union and single market.

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For those in favour of a definitive Brexit, however, the deal fails to deliver on a clean break with the European project.

On Sunday, May defended the terms of the agreement, saying the deal “unlocks a bright future for the UK” and claws back control of “borders … money … [and] laws” from Brussels.

When they (the British people) look at this deal they will see it is a good one for our country and that it is in the national interest for everyone to get behind it,” May said in a statement.

“The British people don’t want to spend any more time arguing about Brexit. They want a good deal done that fulfils the vote and allows us to come together again as a country,” she added.

The UK is due to leave the EU on March 29, 2019.

‘A bad deal for the country’

The leader of Britain’s main opposition Labour Party, Jeremy Corbyn, said in a statement on Sunday he would oppose the brokered agreement during next month’s parliamentary vote.

“This is a bad deal for the country. It is the result of a miserable failure of negotiation that leaves us with the worst of all worlds. It gives us less say over our future, and puts jobs and living standards at risk,” Corbyn said.

“Labour will oppose this deal in parliament. We will work with others to block a no deal outcome, and ensure that Labour’s alternative plan for a sensible deal to bring the country together is on the table,” he added.

The Democratic Unionist Party, whose votes from Northern Ireland have helped May to govern since she lost her majority in a snap election last year, also said it would try to block May’s Brexit deal.

Party officials have described it as “pitiful” – partly because it binds the UK to many EU rules it will no longer help set and partly as the DUP fears it could weaken the province’s ties to Britain.

Wrangling over how to keep open Northern Ireland’s land border with EU member state the Republic of Ireland without creating potentially disruptive customs barriers dogged much of the 18 months of negotiations between UK and EU officials.

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The deal reached allows for a “backstop” to be set up in order to prevent a hard border being established if no trade deal is sorted out during the so-called “post-Brexit transitional period” between March 29, 2019 and December 31, 2020.

Under the terms of the agreement, the whole of the UK will remain in a customs union with the EU “unless and until” the bloc agrees there is no prospect of a return to a hard border.

On Sunday, DUP leader Arlene Foster said the party would review her party’s arrangement with May’s ruling Conservative Party-led government if parliament approves the deal.

“If it came to the situation that parliament did decide, and there’s no evidence that they’re going to, but if they did decide to back this deal then obviously we would have to review the confidence and supply agreement,” Foster told the BBC.

Irish Prime Minister Leo Varadkar, meanwhile, backed May to get domestic sign off on the agreement.

“The alternative is a no deal, cliff edge Brexit which is something, of course, that we all want to avoid,” Varadkar said on Sunday.

“Any other deal really only exists in people’s imaginations,” he added.

If parliament rejects May’s plan, the government will have 21 days to set out how they intend to move forward.

In such a scenario, the UK leaving the EU without a deal, a renegotiation of the existing proposal, holding a general election or even staging a second referendum on EU membership would all become possible outcomes.

Both May and EU leaders have described the current agreement as the “best” and “only possible” deal.

If British legislators back it, then an EU Withdrawal Agreement bill will later be introduced to parliament. That will then need to be ratified by parliamentarians before it can proceed to a European Parliament vote and, if that’s successful, an eventual EU Council approval process.