BRUSSELS — British Prime Minister Boris Johnson’s career of disdain for the European Union was a thing of the past on Thursday as he and the bloc’s leaders celebrated their tentative Brexit deal. He now faces an opponent closer to home: his own Parliament.

With the ink barely dry on the proposal and Johnson still happily backslapping EU leaders at a summit in Brussels, a chorus of British party leaders said they would vote against the deal. Crucially, the Northern Irish party that supports Johnson’s minority government said it would not support the deal, leaving the prime minister uncertain of getting the votes he needs to ratify it.

After an intense week of technical talks and with only two weeks to go until Britain’s scheduled departure on Oct. 31, European Commission President Jean-Claude Juncker broke the tension with a tweet Thursday morning: “We have one! It’s a fair and balanced agreement for the EU and the UK and it is testament to our commitment.”

Johnson tweeted that the two sides had struck a “great new deal” and urged U.K. lawmakers to back it in a special session being held Saturday — the first time since the 1982 Falklands War that the House of Commons has sat on a weekend.

European leaders unanimously endorsed the tentative deal on Thursday, formally sending it to the British Parliament.

But Johnson will face a struggle to get his deal approved. All the major opposition parties condemned the agreement, and Johnson’s key Northern Irish ally also said its lawmakers would oppose it.

Labour Party leader Jeremy Corbyn, Scottish First Minister Nicola Sturgeon and Brexit party leader Nigel Farage all announced they could not back the deal.

“It seems the prime minister has negotiated an even worse deal than Theresa May’s, which was overwhelmingly rejected,” Corbyn said, referring to Johnson’s predecessor who failed to get her agreement through Parliament.

Many opposition lawmakers want to oppose the deal and then seek to delay Brexit while new terms are negotiated.

But Juncker ruled out any new postponement, leaving British lawmakers with a simple choice: deal, no deal or revoke Brexit.

“If we have a deal, we have a deal, and there is no need for a prolongation,” he said. The ultimate decision on any extension though does not rest with Juncker. It’s a decision for the other 27 EU countries.

Johnson’s 10 Downing Street office put it even more succinctly with the mantra: “New deal, no deal but no delay.”

EU Brexit negotiator Michel Barnier said Johnson “told President Juncker this morning that he believed he was able to get the deal approved” in Parliament, adding that Johnson said he was “confident about his capacity to convince a majority.”

Johnson began his career as a Brussels-based journalist spinning exaggerated tales of EU excess for British readers, and as a politician helped lead the campaign to take Britain out of the EU, but that acrimony was nowhere in sight on Thursday.

Instead, relief was palpable in Brussels as leaders happily mingled in the summit room of Brussels’ Europa building, which has all too often been ground zero for European crises. Johnson cheerily saluted French President Emmanuel Macron, who responded with the heartiest of handshakes.

The jubilation soon turned bittersweet, with EU leaders bemoaning the impending loss of a major member state — a military, economic and diplomatic juggernaut that had joined 46 years ago.

At the same time, they were clearly delighted to end the ugly recriminations and insults of months of Brexit bickering.

“I’m satisfied that we were able to find a deal, but I’m sad about the fact that Britain is leaving the European Union,” Juncker said.

The pound hit a five-month high against the U.S. dollar on news of a Brexit deal, then sank back as traders heard Johnson’s Northern Irish allies were still unhappy with the way the deal handles the border between EU member state Ireland and Northern Ireland, which is part of the U.K. — long one of the thorniest issues of any exit.

The new deal is broadly similar to the 585-page withdrawal agreement that May hammered out, with the only major changes on the Irish border issue.

The agreement must also still be formally approved by the bloc and ratified by the European Parliament. That could happen as early as next week when it meets in Strasbourg.

While British leaders kept their legislators at arms’ length during the talks, EU negotiator Barnier has kept the EU parliament fully involved and the final approval is expected to be little more that a rubber stamp.

The deal agreed Thursday will be legally binding if approved — but it doesn’t cover the all nitty gritty of future relations between the U.K. and the EU. It merely lays out the terms for withdrawal, while leaving the details of trade and other issues to future negotiations. The only issues that leaders felt they couldn’t put off and had to hammer out ahead of a U.K. exit were the thorniest ones: how to address the Irish land border and the rights of British and EU citizens living in each other’s territories.

The deal also gives the two sides a grace period to work out those details by keeping relations as they are until the end of December 2020.

The key hurdle was finding a way to keep goods and people flowing freely across the Irish border after Brexit. That invisible, open border has underpinned the region’s peace accord and allowed the economies of both Ireland and Northern Ireland to grow.

Johnson insists that all of the U.K. — including Northern Ireland — must leave the bloc’s customs union, which would seem to make border checks and tariffs inevitable.

Barnier said the deal “squares this circle” by leaving Northern Ireland inside the EU single market for goods — so border checks are not needed on the land border on the island of Ireland. Instead, customs checks will be carried out and tariffs levied on goods entering Northern Ireland that are destined for the EU.

That effectively means a customs border in the Irish Sea between the islands of Ireland and Great Britain — something the British government long said it would not allow and something Northern Ireland’s Democratic Unionist Party vehemently opposes.

DUP leader Arlene Foster and the party’s parliamentary chief Nigel Dodds said they “could not support what is being suggested on customs and consent issues,” referring to a say the Northern Irish authorities might have in future developments on the border.

But the EU has compromised, too, by allowing Northern Ireland special access to its single market. And the deal gives Northern Ireland a say over the rules, something that was missing from May’s previous rejected agreement. After four years, the Northern Ireland Assembly would vote on whether to continue the arrangement or end it.