David Cameron wants to renegotiate the terms of the UK's membership ahead of a referendum by the end of 2017. He has said he will campaign for Britain to remain in the EU if he gets the reforms he wants.
What is Britain looking for?
Mr Cameron said he did not want to reveal full details of his negotiating hand before serious discussions get under way.
His four key objectives are:
- Economic governance: Securing an explicit recognition that the euro is not the only currency of the European Union, to ensure countries outside the eurozone are not materially disadvantaged. The UK wants safeguards that steps to further financial union cannot be imposed on non-eurozone members and the UK will not have to contribute to eurozone bailouts
- Competitiveness: Setting a target for the reduction of the "burden" of excessive regulation and extending the single market
- Immigration: Restricting access to in-work and out-of-work benefits to EU migrants. Specifically, ministers want to stop those coming to the UK from claiming certain benefits until they have been resident for four years. Ministers have reportedly been warned by the UK's top civil servant this could be discriminatory and any limits may be reduced to less than a year. An option of an "emergency brake" to stop the payments for four years is being discussed as a compromise deal
- Sovereignty: Allowing Britain to opt out from the EU's founding ambition to forge an "ever closer union" of the peoples of Europe so it will not be drawn into further political integration. Giving greater powers to national parliaments to block EU legislation.
What else does Mr Cameron want?
The Conservatives want to free business from red tape and "excessive interference" from Brussels and to provide access to new markets through "turbo charging" free trade deals with America and Asia.
They also want trade barriers in the services and digital sectors to be removed to create a truly single market as well as specific protections for the City of London.
They support continued enlargement of the EU to new members but with new mechanisms in place to "prevent vast migrations across the Continent".
The prime minister has said Britain would resist any move towards a European Army and that he wants to free British police forces from EU interference. He has also ruled out Britain joining the euro.
But he has placed less emphasis in recent years on demanding changes to EU social policy, such as the maximum 48-hour working week, agency workers, maternity leave and non-discrimination rules - amid pressure from trade unions to leave such protections intact.
When will the referendum be held?
The official UK government position is that a referendum will be held by the end of 2017 but Downing Street sources have said: "If we can do it earlier we will." it is widely expected to be held in June or July if a deal is reached at the EU summit in February.
What about the European Court of Human Rights?
The Conservatives will attempt to repeal the Human Rights Act, which requires UK courts to treat the European Court of Human Rights as setting legal precedents for the UK, in favour of a British Bill of Rights.
The European Court of Human Rights is not a European Union institution. It was set up by the Council of Europe (CoE), which has 47 members including Russia and Ukraine.
David Cameron says that if necessary he would back a new law reasserting the power of the UK Parliament over the EU.
What about freedom of movement?
The freedom for people to move around Europe, enshrined in the EU treaties, works in parallel with the other three basic freedoms in the single market: freedom of goods, capital and services. It is a "red line" for other EU leaders, who do not want to see it eroded.
Mr Cameron's decision to seek the four year ban on some UK benefits for EU migrants was a way round it, but a number of EU countries - including Hungary and Poland - say they will not accept anything which discriminates against their nationals.
This has led to reports that the idea of an "emergency brake" to limit numbers when there has been a surge of migrants, might be back on the agenda.
What are David Cameron's welfare demands?
In a speech in November 2014, David Cameron set out measures to reduce the number of EU nationals moving to the UK.
His main proposals were:
- Four year delay for EU migrants wishing to claim in-work benefits, such as tax credits, or seeking access to social housing
- Stopping migrants claiming child benefit for dependents living outside the UK
- Removing migrants from the UK after six months if they have not found work
- Restricting the right of migrants to bring non-EU family members into the UK
- Stopping EU jobseekers claiming Universal Credit
- Speeding up deportation of convicted criminals
- Longer re-entry bans for beggars and fraudsters removed from the UK
- Stopping citizens from new EU entrants working in the UK until their economies have "converged more closely".
- Extra money for communities with high levels of migrants
How have EU leaders reacted?
Jean-Claude Juncker, President of the European Commission, has said he is ready to work with Mr Cameron to "strike a fair deal for the United Kingdom in the EU".
But his spokesman reacted to the publication of Mr Cameron's negotiation objectives by warning that restrictions on benefits for new arrivals to the UK would be "highly problematic" as they affected the "fundamental freedoms of our internal market" and amounted to "direct discrimination between EU citizens".
The main sticking point has been opposition from countries including Hungary and Poland to the idea of the four year ban on claiming some UK benefits. The message from the rest of the EU is that there cannot be a policy which discriminates against other EU nationals.
However David Cameron has said he is willing to consider alternatives that have the same impact on migration flows, while his EU colleagues have also said they want to come up with a deal which achieves the UK's aims.
What if Mr Cameron fails to get the changes he wants?
Mr Cameron has faced repeated calls to say whether he would campaign for Britain's exit from the EU in the referendum if he fails to get what he wants from the renegotiation process. All he has said is that he will "rule nothing out" and he has no "emotional attachment" to the EU.
What do other Conservative MPs want?
Some Conservative MPs want Britain to leave the EU no matter what David Cameron manages to renegotiate. A larger group are likely to want more concessions than Mr Cameron has so far indicated he is prepared to demand.
Senior backbencher David Davis, who was defeated by Mr Cameron for the Tory leadership in 2005, told the Daily Telegraph the majority of the 332 Conservative MPs want Mr Cameron to negotiate an "opt-out" power to stop individual EU laws from applying to Britain. The prime minister has said this would be "impossible" without Britain leaving the EU.
Mr Davis claims as many as 60 Tory MPs would be prepared to rebel and vote for Britain's exit from the EU if the prime minister fails to deliver.
Will there be a free vote for cabinet ministers?
Graham Brady, chairman of the Tory backbench 1922 committee, which represents backbenchers, had urged David Cameron to give all Conservative MPs, including ministers, a free vote in the EU referendum despite Mr Cameron warning in January 2015 that serving cabinet ministers would have to quit the government if they wanted to campaign to leave the EU - if he had secured the reforms to it he wants.
That position has now changed with Mr Cameron agreeing to allow cabinet ministers to campaign - in a personal capacity - on either side of the EU referendum campaign, once his renegotiation is over.
In the 1975 referendum on whether Britain should remain in the European Economic Community, as the EU was then called, Labour Prime Minister Harold Wilson decided to suspend collective responsibility because his cabinet was split on the issue.
Ministers were allowed to vote with their consciences and campaign against each other. The majority of them, including Mr Wilson, joined the Yes camp, but left-wingers, such as Tony Benn, played a leading role in the No campaign. The Yes campaign won easily with 67% of the vote.
Referendum on the UK's future in the European Union
The UK is to have a referendum by the end of 2017 on whether to remain a member of the European Union or to leave. The vote is being proceeded by a process of negotiations in which the Conservative government is seeking to secure a new deal for the UK.