LONDON (AP) — European Union leaders are holding a summit in Brussels today and Friday to hammer out a deal designed to keep Britain in the 28-nation bloc.

The stakes are high and the issues are complex.

Chancellor Angela Merkel said Wednesday that it's in Germany's national interest for Britain to remain in the EU. She also pressed her fellow European leaders to work with Turkey to curb the migrant influx.

In a speech to Parliament, Merkel painted a largely positive picture of chances of an agreement on Britain's EU reform demands, which she said are in many cases "justified and understandable."

EU President Donald Tusk said late Wednesday that the outcome of the talks was still on a knife's edge.

"Frankly: there is still no guarantee that we will reach an agreement," he wrote to government leaders in the wake of his whirlwind tour that took him to five capitals in little more than one day.

In case the talks would still break down, he wrote, "it would be a defeat both for the UK and the European Union, but a geopolitical victory for those who seek to divide us."

On migrants, Merkel made clear that she won't be pushing the contentious subject of new quotas to distribute migrants around Europe.

She reiterated that it would be "laughable" for Europe to approve such quotas when it has barely started to share refugees under existing agreements.

Some of the ins and outs of the UK-EU relationship:

What is the European Union?

Germany, France and four other nations formed the European Economic Community in 1957, determined to banish forever the bloodshed of two world wars.

The grouping became the EU in 1993 and has grown into a 28-nation bloc of more than 500 million people stretching from Ireland to the Aegean Sea, with substantial powers over member states' laws, economies and social policies.

It has its own parliament and central bank, and 19 EU members use a common currency, the euro.

Why is Britain considering leaving?

Britain joined the bloc in 1973, but many Britons feel their island nation — a former imperial power with strong ties to the United States — is fundamentally different from its European neighbors. Anti-EU Britons resent everything from fishing quotas to fruit sizes being decided in Brussels, though supporters say Britain's economy and security are enhanced by EU membership.

The anti-EU view is especially strong in the Conservative Party of Prime Minister David Cameron. It was partly to appease his party that Cameron promised to hold a referendum on EU membership by the end of 2017. The prospect of Britain leaving the EU is known as "Brexit" — short for British exit.

What could get Britain to stay?

Cameron argues that Britain should stay in the EU if he can get changes cutting red tape and giving individual countries more power.

EU leaders at the summit will try to agree on change in four areas: economic governance; competitiveness; sovereignty; and social benefits and free movement. The last section is the most contentious — Britain wants to be able to make immigrants from other EU countries wait for up to four years before they can receive some welfare benefits, notably payments if they have children.

Will EU leaders reach an agreement?

Other EU countries value Britain for its diplomatic and economic clout and because it provides a counterweight to France and Germany. But they get frustrated with its constant demands. European Parliament President Martin Schulz said recently that some EU politicians privately think: "Don't stop a rolling stone. If the Brits want to leave, let them leave."

There is a general desire to help keep Britain, but some sticking points remain. Eastern European nations don't want other countries to be able to use the brake on benefits offered to Britain, while countries in the single currency worry that protections for Britain and other non-euro zone nations unduly advantage to Britain's financial center, the City of London.

What happens next?

If there is a deal in Brussels, Cameron is expected to announce a referendum date as soon as Friday, with June 23 the most likely option.

The referendum's outcome is hard to predict, because there is little precedent — Britain hasn't had a referendum on Europe since 1975.

Opinion polls were notoriously inaccurate about Britain's 2015 election and vary widely. Some show a lead for the "remain" side, while others put "leave" ahead.

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