Future of UK Hangs in Balance as Brexit Polls Open

Polls opened this morning for millions of Britons planning to vote in a historic referendum that could determine whether the United Kingdom remains in the European Union.

The vote on whether to leave the EU - referred to as the Brexit - could have a wide-ranging impact on the global economy as well as the internal politics of the U.K.

Here is everything you need to know about today's vote:

What Is the European Union?It is an economic and political union that has its roots in the European Coal and Steel Community, which was established in 1951 and consisted of just six countries. Today the EU has 28 members.

It can impose laws on member countries, but it is up to individual nations to implement them.

In 1973 the U.K. joined what was then called the European Community. The U.K. is not part of the eurozone and thus has its own currency (the pound) rather than the euro, which was adopted across much of the continent. It is also not part of Schengen Area, meaning it has retained greater sovereignty over its border controls than participating nations have.

How Will the UK Referendum Work?U.K. and commonwealth citizens age 18 or older and living in the U.K. are eligible to vote. Those who have been living overseas for no more than 15 years are also eligible.

Results will be declared in 382 voting offices around the U.K., and the final nationwide result will be announced in Manchester, England, on June 24.

The vote is not legally binding. If the "leave" side wins, it will still be up to Parliament to repeal the 1972 act that authorized the U.K. to join the bloc.

Why Is the Referendum Being Held?David Cameron, as the head of Conservative Party, promised to hold a referendum while campaigning for re-election in the 2015 general election, in response to growing Euroskeptic pressure in his party and across the country.

What Will Happen If the UK Leaves the EU?The immediate impact of a Brexit is unclear because the U.K. would have to negotiate a deal with the EU. That would take about two years. During that time, the U.K. would remain in the EU but would not be able to participate in any decision-making.

Scotland - which held a referendum of its own in September 2014 on whether to leave the U.K., with 55 percent voting against independence - is largely pro-EU. A Brexit could result in Scots revisiting the independence question.

Why Leave the EU?Immigration is cited as a big concern for many who are against EU membership. According to the "leave" camp, some 250,000 migrants from other EU countries enter the U.K. each year.

Brexit proponents also argue that 350 million pounds a week is sent to the EU and that only 5 percent of businesses in the U.K. export to Europe but 100 percent are burdened by EU-imposed regulations.

Sovereignty is another factor, with those in favor of a Brexit saying they want to take charge of borders and be able to control migration flows.

Why Remain in the EU?Those who want the U.K. to stay in the EU argue that the benefit of being in the single market - the world's largest free-trade zone - is worth $127.7 billion annually.

It has been estimated that more than 3 million U.K. jobs are linked to trade with the EU and that some 950,000 positions could be lost by leaving.

Being a part of the EU yields other benefits, say those in favor of remaining, such as cheaper and easier travel, continentwide crime-fighting solutions and strengthened labor and environmental protection laws.

Pro-Europe voices have suggested that Britain has a stronger voice on the world stage by remaining in the EU. Moreover, the bloc has succeeded in its most important task since being formed in the shadows of World War II: securing peace.

What Do the Experts Say?Most experts say that the outcomes of leaving would depend on the agreement that the U.K. would negotiate with the EU. "It depends if we keep the free movement of labor," said Ian Preston, an economics professor at University College London. "If we leave the economic area and get [World Trade Organization]-type agreements instead, that is the most damaging economically. We will see a decrease in trade and foreign direct investments. That means lower GDP, which means lower tax receipts and an increase in taxes or borrowing."

What Do the Polls Say?An early lead for the "in" campaign has turned in recent weeks, with the "leave" camp now ahead, according to some surveys. But many are suspicious about British polls.

The latest BMG Research poll found that 62 percent of voters will "definitely" cast a ballot.

Is It Really Just About the EU?"It's not just about the EU. It's also driven by domestic politics in the U.K.," said Tim Oliver, a Dahrendorf fellow for Europe-North American relations at the London School of Economics. "It's about kicking the government, it's about Britain's identity, it's about globalization fears, about party politics, about London versus the rest of the U.K. and about the political and economic elite versus people who feel disenfranchised."

The U.K. has often been dubbed the awkward partner of Europe. "The U.K. is historically a Euroskeptic country," Matt Goodwin, a politics professor at the University of Kent, told ABC News. "There's no doubt that there will be a strong 'leave' vote, and that will ensure hostility to Brussels and Westminster elite will continue. There is almost no chance to find unity, at least in the current generation, about the European question."