UPDATE:

British Prime Minister David Cameron says he has negotiated a deal with European leaders to change the terms of Britain's membership in the European Union.

European Council President Donald Tusk said on Twitter that the countries had unanimously agreed to the new deal.

Cameron had been seeking concessions for his country so that he can encourage Britons to vote for continued EU membership in a referendum expected in the coming months.

He posted on Twitter that he would recommend the new deal to the U.K.'s Cabinet Saturday. After the ministers are briefed, "they will be free to campaign for either side in the referendum, which has been promised by the end of 2017 but is expected in June," the BBC reports.

Here are some of the key elements of the deal, according to documents seen by Reuters:

Our original post, with more on the debate, is below:

The prime minister promised to hold that referendum in the runup to last year's election in the U.K., as he tried to fight off a threat from a far-right anti-EU party that threatened to take votes from Cameron's Conservative Party.

Having made that promise, Cameron now has to negotiate enough concessions from the other 27 countries in the EU to assuage the "euro-skeptic" wing of his own party and win over the British public.

He faces opposition from other EU members who don't want to make exceptions for a particular member state. He's also been criticized by EU opponents in Britain, who say he is not asking for enough — or who the EU no matter what.

So what does Cameron want? Last November the prime minister set out his objectives in a letter to the president of the EU Council, Donald Tusk. He emphasized four points:

Sovereignty: Cameron wants to end Britain's commitment to work toward an "ever closer union" — a founding principle of the EU.

Immigration: The prime minister wants to make the U.K. less attractive to immigrants by restricting the benefits foreign workers from other EU countries can receive in Britain.

Competitiveness: Cameron wants a plan to reduce the amount of legislation and regulation coming out of the EU's headquarters in Brussels and lift what he calls "the burden [of] existing regulation" from business.

Economic Governance: Britain wants to make sure that decisions made by the core of nations that use the euro as their currency will not adversely affect the countries that have retained their own currency, such as the British pound.

Euro leaders have their own dilemma. They desperately want Britain to stay in the EU and fear the union would be severely damaged or even break up if the U.K. were to leave. So they understand that they have to offer Cameron some concessions to stop that from happening. But they also worry that if they do that, other countries will demand changes in their own relations with the EU, and the situation will spin out of control.

What's the U.S. interest in this? President Obama has made it clear that his administration wants Britain to stay in the EU, because Washington likes having a close ally such as the U.K. inside the group, and foreign policy experts fear the instability that might follow if Britain were to leave.

So the negotiations in Brussels today are being watched with some anxiety on both sides of the Atlantic. Some kind of deal is likely that will enable Cameron and his ministers to campaign in favor of Britain staying in the euro ahead of the referendum.

But Cameron's deal might not influence that vote as much as people think. Many British people have a viscerally negative view of the European Union, and are unlikely to be swayed by technicalities being discussed today. So the question for the next three months is, will the British public vote with their hearts ... or their heads?

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