Prime Minister David Cameron said Britain would vote June 23 on whether to stay in the European Union. (Luke Macgregor/Bloomberg News)

Having persuaded 27 fellow European Union leaders to do a deal to save Britain’s E.U. membership, Prime Minister David Cameron faced an insurrection at home on Saturday as his government emerged divided over whether to back a Brexit.

In a rare Saturday morning cabinet meeting — the first since the Falklands War in 1982 — Cameron attempted to rally his senior ministers to the cause of keeping the United Kingdom a part of the European Union when the country votes in June.

The meeting came hours after the prime minister inked a deal in Brussels with his E.U. counterparts that he said would dramatically improve British relations with the bloc. The agreement featured concessions in various areas, including currency protections and immigration, and it came together only after two days of round-the-clock talks.

But with a referendum campaign now underway in Britain, there were major defections from the government’s senior ranks, reflecting bitter divisions in the prime minister’s Conservative Party over the country’s membership in the E.U. Polls show that voters as a whole are almost evenly split.

Speaking in front of 10 Downing Street on a gray Saturday, Cameron announced that Britons will decide the issue on June 23, giving both sides four months to try to persuade a majority of voters. Cameron had first promised the referendum in 2013, bowing to a strong current of Euroskepticism that has run through British politics for decades and is unequaled anywhere else on the continent.

A British departure would be a first for the bloc, and it could imperil the union’s future by empowering anti-E.U. forces across the continent.

The stakes are high for Britain, as well.

[What’s a ‘Brexit’? A guide to Britain’s E.U. drama]

“We are approaching one of the biggest decisions this country will face in our lifetimes,” Cameron said Saturday.

The prime minister announced that a majority of his cabinet was recommending that the British public vote to stay in, and he argued that a departure — popularly known as a Brexit — would damage Britain by depriving the country of vital partners.

“Leaving Europe would threaten our economic and national security,” he said.

But only minutes after the prime minister spoke, a half-dozen cabinet ministers announced they would defy Cameron and side with “out.”

Cameron had bucked British political convention by allowing his ministers to choose either side of the E.U. debate, rather than demanding loyalty.

Saturday’s defections were not a surprise; six of the ministers have been sharply critical of the E.U. in the past. But their stance reflects just how politically divisive the referendum is likely to be, cutting across party lines.

Among the defectors — dubbed #TheSecessionistSix on Twitter — is Justice Secretary Michael Gove, an influential Tory and one of Cameron’s closest friends.

In a lengthy statement released Saturday afternoon, Gove said that he was anguished at the idea of opposing the prime minister, whom he credited with launching his political career. But he said he could not ignore his belief that the United Kingdom would be “freer, fairer and better off outside the E.U.”

The union, Gove wrote, is a relic of the 1950s and 1960s that “is now hopelessly out of date.” It is also, he argued, fundamentally antidemocratic.

“Laws which govern citizens in this country are decided by politicians from other nations who we never elected and can’t throw out,” he said.

Cameron said he was “disappointed but not surprised” by Gove’s decision.

[Has Prince William entered Britain’s debate over leaving Europe?]

Other top government officials opted for “in,” including George Osborne, Britain’s chancellor of the exchequer, and Home Secretary Theresa May.

May, a hard-liner with Euroskeptic leanings who was at one time considered a possible Brexit supporter, released a statement Saturday announcing she was for “in.”

She said the decision was “for reasons of security, protection against crime and terrorism, trade with Europe, and access to markets around the world.”

London Mayor Boris Johnson, a leading Conservative who covets Cameron’s job, has also toyed with supporting the “out” campaign.

He did not immediately show his hand Saturday, and the BBC reported he was unlikely to announce his decision until Sunday at the earliest. Johnson would give the “out” movement the sort of charismatic and broadly popular leader it lacks.

Compared with the Conservatives, the center-left Labour Party is less divided over the issue, with most of the party’s elected officials supporting E.U. membership.

Labour leader Jeremy Corbyn described Cameron’s E.U. renegotiation Saturday as “tinkering.” But he nonetheless said his party would campaign to stay in the E.U. because “it brings investment, jobs and protection for British workers and consumers.”

The political leanings of Britain’s newspapers were on vivid display Saturday morning, with right-wing papers dismissing Cameron’s Brussels deal and left-leaning ones praising it.

The progressively minded Guardian said the renegotiations were “substantive not superficial,” while the Rupert Murdoch-owned Times of London pronounced that “from the land of chocolate, Cameron was always destined to bring back fudge.”

[Europe wants Britain to stay in the E.U., but not at any cost]

Analysts suggested that Cameron had won a better deal than many expected but generally played down the effect of European concessions. The prime minister won a British exemption from Europe’s goal of “ever-closer union,” a national veto on E.U. laws, protections for countries that do not use the euro and “an emergency brake” to limit benefits paid to immigrants from within the E.U.

Cameron trumpeted the latter concession as a chance to limit net migration to Britain, which is at an all-time high. But experts have cast doubt on the claims, noting that most workers do not come to Britain for government benefits.

The Center for European Reform concluded in a briefing note that Cameron’s “package of reforms will sway few voters, so he must now make the case for the E.U. itself.”

“Cameron’s best chance of success,” the think tank said, “is to shift the debate onto more lofty terrain, away from arguments about banking safeguards and migrants’ benefits towards a contest over how to secure Britain’s interests in Europe and the rest of the world.”

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