BRUSSELS (AFP) - European Union leaders meet on Thursday (Feb 18) for what promises to be a tense two-day summit to discuss new terms for Britain's membership of the EU.
Here is a summary of what it means to be a member of the EU and how Britain stands apart:
HOW DID THE EU BEGIN?
The EU traces its origins to the 1950s, when half a dozen countries in continental Europe launched the first moves of economic integration after the devastation of World War II.
The European Economic Community (EEC) emerged from this process in 1958, with Belgium, France, Germany, Italy, Luxembourg and the Netherlands as the founder members.
Jealous of an economic boom enjoyed by France and Germany, Britain joined the bloc in 1973 after overcoming fierce objections in Paris.
In 1992, the Treaty of Maastricht expanded integration, creating the European Union and laying the foundations for the single currency, the euro - the project that is proving to be one of the most contentious issues in Britain's continued membership.
After years of gradual expansion, membership exploded, with the addition of nearly a dozen former communist states in 2004 and 2007. Croatia was the latest to join in 2013, bringing the club to its current 28 members.
ARE ALL EU MEMBERS EQUAL?
In the EU, not all members carry the same political weight nor obey the same rules. Whether concerning the euro, the EU Budget or the free movement of people, the European project is riddled with exceptions.
The Schengen passport-free zone is one of its pillars, enshrining the fundamental right to free movement.
Britain is the only EU country to have opted out of both Schengen and the euro, choosing to control its own borders and currency. Denmark also is under no obligation to eventually join the euro.
All other EU members are expected to adopt the single EU currency, although several countries, including Sweden and Poland, are openly reluctant to do so after the turbulence of the euro zone debt crisis.
WHO CONTRIBUTES TO ITS BUDGET?
As for the EU Budget, which Britain got cut back in 2014, member states contribute on the basis of their per-capita wealth and population size. The current 2014-2020 Budget is worth nearly a $1 trillion euros (S$1.56 trillion).
Germany, France and Britain are the biggest contributors, with Poland, Hungary and Greece the biggest net recipients.
The Budget comprises three main parts - the Common Agricultural Policy (CAP), which covers farm aid and accounts for about a third of spending; Cohesion Funds, which help poorer member states catch up with their peers; and programmes to boost innovation and jobs.
Then British Prime Minister Margaret Thatcher in 1984 famously secured a rebate on Britain's contribution, on the argument that Britain, with a small farming sector, received much less CAP funding than its peers. The British rebate has since become an article of faith, with London hostile to any attempt to cut it.
HOW DOES A COUNTRY LEAVE THE EU?
The Treaty of Lisbon, agreed in 2009, introduced an EU exit clause, allowing member states a legal path to leave the bloc voluntarily.
A country that wants out of the EU must notify the European Council, the paramount decision-making body which groups the governments of the 28 member states. Any withdrawal would then be negotiated in a process likely to take months.
Officials warn that an exit by Britain, a major economy that also hosts the City of London financial hub, would be tortuous.
ARE ANY ADDITIONS EXPECTED?
The EU has put the brakes on new members, ruling out any additions until 2020 ever since the expansion to the east proved too much for the current institutions to handle.
Turkey, along with Serbia, Montenegro, Macedonia and Albania, is among the current applicants. Britain notably supports Turkey's membership bid, but many others are bitterly opposed.