LONDON — Call it the British wish list: an array of demands, aspirations and proposals sent to its partners in the European Union on Tuesday that clarifies battle lines in an intensifying debate over whether Britain should quit the bloc.
It is the first time the British government has spelled out how it wants to change its relationship with the 28-member European Union, which has faced a year of upheavals that have tested E.U. unity and prompted questions about how much power should rest with officials in Brussels.
But the document also seemed to fall short in the eyes of those most eager for Britain to strike out on its own.
Prime Minister David Cameron has long said that he favors staying in a “reformed” E.U., but he had faced criticism from other E.U. officials, who said Britain was holding its cards too close to its chest and not being explicit about the changes it would like to see.
Cameron sought to remedy that Tuesday with the letter to Donald Tusk, president of the European Council, which effectively kicked off formal negotiations about E.U. reform.
Cameron has promised to hold a referendum on Britain’s membership in the E.U. before the end of 2017, the outcome of which could fundamentally reshape both Britain and the E.U.
Mujtaba Rahman, an analyst with Eurasia Group, said Cameron published the letter now in part because he wants to have produced something tangible ahead of a summit in December with E.U. leaders. Rahman said that he did not expect an agreement then but that Britain nonetheless wanted to “take the first bite of the apple” before the meeting.
As expected, Britain’s demands cover four broad themes — efforts to increase the E.U.’s competitiveness, a bigger role for national parliaments, safeguards for countries not using the common euro currency and — perhaps most controversial — the curbing of welfare payments to other E.U. citizens living in Britain.
Euro skeptics dubbed the proposals cosmetic and relatively modest.
Nigel Farage, the leader of the U.K. Independence Party, which wants Britain to leave the bloc, said on Twitter that it was “clear that Mr. Cameron is not aiming for any substantial renegotiation.”
Jacob Rees-Mogg, a member of Cameron’s Conservative Party, said the plans were “pretty thin gruel.”
In a major speech in central London, Cameron said sweeping E.U. changes might spur the British public to vote to stay in the bloc. But he also warned that he would consider campaigning to leave the E.U. if Britain’s concerns fall on “deaf ears.”
Achieving E.U. reform would be “hard work” but still “mission possible,” he said.
The referendum would be “perhaps the most important decision British people will have to take at the ballot box in our lifetimes,” he said.
At the beginning of the year, the chances of Britain’s leaving the E.U. seemed slim, but polls now show the gap narrowing, with marginal differences between the “remain” and “leave” sides.
Analysts say the migration crisis engulfing Europe has made it more difficult for the British government to argue the merits of remaining in the E.U.
Robin Niblett, director of Chatham House, a London-based think tank, said that although the E.U. is preoccupied with other issues — including a massive influx of migrants and Greece’s troubled economy — it is keen that Britain remain a member.
A “Brexit,” he said, would be “disastrous — it would unleash the potential that people could leave the E.U.” He said other E.U. member states “don’t want the additional cataclysmic effect that a Brexit would have on the cohesion of the E.U.”
But Cameron is expected to face resistance to some of his demands, notably his push to limit benefits for other E.U. citizens living in Britain. The freedom of E.U. citizens to live and work in any member state is a key tenet of the E.U. project, and anything that appears to restrict that freedom of movement could be challenged.
In his letter, Cameron proposed that E.U. citizens coming to work in Britain not qualify for some benefits until they have lived in the country for four years.
European Commission spokesman Margaritis Schinas said that some of Cameron’s demands were “highly problematic as they touch upon the fundamental freedoms of the internal market,” the Reuters news agency reported.
He said that “direct discrimination between E.U. citizens clearly falls into this last category.”
German Chancellor Angela Merkel struck a more collegial tone at a news conference Tuesday, saying, “There are some difficult points and less difficult points. But if one has a spirit of wanting to solve this, then I’m reasonably confident that this can work out.”
Even if Cameron were to get all of the concessions he wants, there is no guarantee that voters will favor remaining part of the bloc.
“When we poll British voters, it’s clear they want to go beyond benefit changes. They want fundamental reform on issues of free movement. Voters don’t simply want in-work benefits curbed for migrants, they want fewer migrants — and that’s the awkward reality for David Cameron,” said Matthew Goodwin, a politics professor at the University of Kent.
Cameron also stressed that there would be no second referendum on E.U. membership.
“If we vote to leave, then we will leave. . . . This is our only chance to get this right for Britain and for the whole E.U.,” he said.