British Prime Minister David Cameron on Tuesday officially outlined his government's demands for European Union reforms, saying they aren't "Mission Impossible."

Cameron said the European Union must agree to "irreversible changes" — and limit freedom of movement by allowing the U.K. to restrict benefits for migrants from other member states — as part of the broad reform package he seeks.

Britain will hold a referendum by the end of 2017 on whether to leave the 28-nation EU. Cameron says he wants to stay in, provided he can secure greater autonomy for Britain. He called the vote the most important Britons will face "in our lifetimes."

Cameron outlined his demands Tuesday in a speech in London and a letter to European Council President Donald Tusk, marking the formal opening of what is expected to be a difficult campaign for concessions from other European leaders. He admitted that getting the other 27 nations to agree would be a major challenge.

After outlining the demands, the European Commission said some of the issues raised by Cameron are "highly problematic."

Cameron told an audience at the Chatham House think-tank that Britain wants change in four areas, including protection for countries such as Britain that don't use the euro single currency, less red tape and greater power for national parliaments to opt out of rules made by the Brussels-based EU.

"We are a proud, independent nation. We intend to stay that way," Cameron said, stressing that Britain wanted a "clear, legally binding and irreversible" exemption from the EU's commitment to an ever-closer union.

Most contentiously, Cameron said Britain wants to "tackle abuses of the right to free movement, and enable us to control migration from the European Union." He said Britain wants to bar EU migrants from receiving tax credits and other benefits paid to working people during their first four years in Britain.

That is likely to be a tough sell with some EU leaders, who see free movement of labor, as well as of goods, as a cornerstone of the bloc.

"We don't want to destroy that principle," Cameron said. "But freedom of movement has never been an unqualified right."

European Commission spokesman Margaritis Schinas said Tuesday that some of Cameron's proposals for talks to make sure that Britain stays in the EU may be feasible, but insisted others ranged from "difficult to worse."

He specifically latched on to proposals where freedom of movement would be limited by allowing the U.K. to restrict benefits for migrants from other member states.

"Some things which are highly problematic as they touch upon the fundamental freedoms of our internal market, direct discrimination between EU citizens clearly falls into this last category," Schinas said.

Raf Casert contributed to this report from Brussels.