British Prime Minister David Cameron is shown at a conference in Brussels in December. (Yves Herman/Reuters)

A draft agreement aimed at keeping Britain in the European Union was unveiled Tuesday, opening two weeks of intense negotiations and possibly paving the way for an early referendum on E.U. membership in June.

Donald Tusk, the president of the European Council, sent the proposal to all 28 members of the bloc after what he called “crucial” talks with British Prime Minister David Cameron.

“To be, or not to be together, that is the question . . . My proposal for a new settlement,” tweeted Tusk, channeling his inner Shakespeare and linking to the proposal.

Cameron told the BBC that the draft agreement offered “real progress” on his four broad demands, but added that there was still “detail to be nailed down.”

But some critics dismissed the draft provisions for not going far enough to address key issues such as reducing the immigration flow from the rest of the European Union.

The deal “smells funny,” said the Conservative lawmaker and leading Euroskeptic Steve Baker.

The Daily Mail’s front-page headline on Tuesday read: “Is that it then, Mr Cameron?”

Cameron is trying to renegotiate the terms of Britain’s relationship with the European Union ahead of a promised in-or-out referendum before the end of 2017. If the deal on the table is approved at a Brussels summit to be held Feb. 18-19, it could lead to a referendum as early as June.

On the all-important issues of immigration and welfare benefits, an “emergency brake” was offered that would allow Britain to curb benefit payments to E.U. citizens for up to four years shortly after a referendum. But it remained unclear exactly how such a lever would operate.

Cameron’s Conservative government wants to dramatically slash the number of migrants arriving from other E.U. nations, but one of the fundamental principles of the bloc is the right to live and work in other member states. Cameron says that restricting benefits will reduce the incentives to move to Britain.

Welfare changes were not the only ones proposed.

The package also addressed an increase of national sovereignty over E.U. decision-making, safeguards for countries not using the common euro currency, and measures to make the E.U. more competitive.

“To my mind it goes really far in addressing all the concerns raised by Prime Minister Cameron,” Tusk wrote in a letter. “The line I did not cross, however, were the principles on which the European project is founded.”

One of the proposals includes a “red card” veto that would give national governments the ability to block E.U. legislation with the support of at least 55 percent of the countries in the bloc.

Matthew Elliott, chief executive of the Vote Leave campaign, called the proposals “gimmicks” and said the high threshold for the “red card” proposal makes its use “incredibly unlikely.”

Lucy Thomas, a spokeswoman for the campaign group Britain Stronger In Europe, tweeted that the “red card” would “give U.K. more power to block unwanted E.U. rules.”

Cameron has said he would like to see an agreement at the Brussels summit but has stressed he won’t rush into it if he doesn’t believe he has secured a good deal. But many think Downing Street prefers to hold the referendum sooner rather than later.

Mujtaba Rahman, a Europe analyst with the Eurasia Group, wrote in a recent commentary to his clients that a referendum held after June “would open the door to a host of negative forces including the seasonal uptick in migrant flows, French and German electoral cycles and a dissipation of political momentum.”

He also said that it would risk “higher-level defections among cabinet ministers who currently appear ready to back a reasonable deal.”

Cameron is allowing Euroskeptic ministers to campaign for an exit from the European Union, or “Brexit” as it is known, but they are not allowed to speak out until a deal is finalized.

It is unclear whether all of the 27 other E.U. members will ­rubber-stamp the deal. While ­eager for Britain to remain in the club, their support is not unconditional.

“This negotiation with the U.K. cannot be at any price,” Pervenche Berès, a French member of the European Parliament, told the BBC. “To help Cameron to win this referendum, we cannot do it at the price of destroying all the rest of what is our thinking.”

Poland and other East European countries have already signaled their resistance to the emergency-brake proposal, while France is jittery about the proposals over the protection of non-euro-zone countries.

Not only is the future of the E.U. at stake — no country has ever left the bloc — but existential questions about Britain itself have surfaced.

Nicola Sturgeon, Scotland’s first minister, recently said that it was “highly likely” that a vote to leave the E.U. would trigger a second Scottish referendum on independence. Last year, Scots voted to remain with the rest of the United Kingdom — England, Wales and Northern Ireland.

Opinion polls predict a tight race on the E.U. question. A recent poll by YouGov showed that 38 percent would vote to “remain,” and 42 percent would vote to “leave” the European Union.

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