The proposal, drafted by the European Council president, Donald Tusk, addressed all the issues that Prime Minister David Cameron had insisted be revisited if he was to campaign to keep Britain in the union. But it remained vague on some crucial points, and in any case was unlikely to sway those most committed to Britain’s exit from the bloc.
Mr. Cameron’s task in the months before the referendum is to rally enough supporters of continued membership and win over enough of those on the fence to avert a vote to leave, a choice that many predict could have global ramifications.
Written after weeks of diplomacy, the dense texts still need to be approved by leaders of the other 27 members of the bloc, who, along with Britain, will meet for a crucial summit meeting in Brussels this month. A deal there could pave the way for a British referendum as early as June.
Not only would a British exit from the bloc cause acute economic uncertainty in and beyond Britain, it could also trigger an existential crisis for the union, which has struggled in vain to react coherently to a growing wave of migration from the Middle East and elsewhere.
Mr. Cameron has said that he wants to negotiate a “better deal” from the bloc, one that would then allow him to campaign for the country to stay. On Tuesday, speaking in Chippenham, England, Mr. Cameron called the new plan a “very strong and powerful package,” adding that, while there was no final agreement and more work was needed, “strong, determined and patient negotiation has achieved a good outcome for Britain.”
Immediate reaction was divided along well-established lines, with critics of the European Union denouncing the proposals as insubstantial.
Nigel Farage, leader of the U.K. Independence Party, called them “truly pathetic.” Steve Baker, a Conservative Party lawmaker who also wants Britain to quit the bloc, said that “nothing in it would stand up to serious scrutiny.”
But Carolyn Fairbairn, director general of the Confederation of British Industry, a business lobby group, described the offer as “an important milestone on the way to a deal that could deliver positive changes to the E.U. that will benefit not just the U.K., but the whole of Europe.”
Significantly, one senior euroskeptic figure in Mr. Cameron’s Conservative Party, Home Secretary Theresa May, signaled cautious support for the plan on Tuesday, describing it a “basis” for a deal.
The most delicate issue on the table was Mr. Cameron’s call for the right to restrict welfare benefits for non-British citizens of European Union countries, namely by limiting access to “in work” payments that typically supplement the earnings of low-wage employees.
These curbs could apply for up to four years, and the documents published on Tuesday state that the scale of immigration into Britain would justify them. However, it also stipulated that there would need to be a final agreement among the 28 nations for the restrictions to kick in.
Plans would also be drawn to reduce the “child benefit” payments to workers whose children have not accompanied them to Britain. This plan would involve Britain paying a lower amount based on costs in the nation where the child lives.
Mr. Cameron’s welfare proposals were seen by some nations, most notably in Eastern Europe, as a breach of the principle that all European Union citizens should be treated equally.
In a letter accompanying the release of the documents, Mr. Tusk defended his attempt to balance British demands against the sensitivities of other countries. “To my mind, it goes really far in addressing all the concerns raised by Prime Minister Cameron,” Mr. Tusk wrote. “The line I did not cross, however, were the principles on which the European project is founded.”
In a Twitter post, Tomas Prouza, the Czech minister for European affairs, described the mechanism as “acceptable” but said there would be a crucial debate over how long the restrictions would apply.
Mr. Tusk’s proposals also offered assurances to Mr. Cameron that a treaty commitment to “ever closer union among the peoples” of Europe would not bind Britain to the goal of political union.
Instead the proposals argued that this was “compatible with different paths of integration,” does “not compel all Member States to aim for a common destination” and allows for “an evolution towards a deeper degree of integration among the Member States that share such a vision of their common future, without this applying to other Member States.”
Another proposal ensured safeguards for the large financial sector in Britain, which decided to keep the pound rather than adopt the euro. The British government worries that, as the 19 nations that use the single currency integrate further, rules might be skewed against European Union nations that do not.
Offering such guarantees to Britain is sensitive in some eurozone nations, such as France.
In another concession, European Union legislation could be blocked if enough national parliaments oppose a measure, though critics doubt that that would be easy to deploy.
Mr. Cameron’s enthusiasm for the plan is crucial because those who want Britain to stay in the bloc believe his opinion will prove decisive with the public in a referendum.
He argues that, inside Europe’s single market, but outside its euro single currency and the passport-free Schengen travel zone, Britain could have the “best of both worlds” if it succeeds in its negotiation.
Britain voted in 1975 to stay in what was then called the European Economic Community, which it had joined two years earlier, but has held no plebiscites on European issues since.
On Tuesday Mr. Cameron said that, providing that the deal is reached, he would not argue that “the European Union is now a perfect and unblemished organization” but that “on balance, Britain is better off” inside it.