David Cameron has promised a referendum on whether Britain should remain in the European Union by the end of 2017. Here is a summary of the key arguments for and against British membership.
Are there any viable options for Britain leaving the EU?
If Britain votes to leave the EU, it will have to negotiate a new trading relationship with what would now be a 27 member organisation, to allow British firms to sell goods and services to EU countries without being hit by excessive tariffs and other restrictions.
Better off out: Britain could negotiate an "amicable divorce", but retain strong trading links with EU nations, say those campaigning for Britain's exit.
Some potential models:
◾ The Norwegian model: Britain leaves the EU and joins the European Economic Area, giving it access to the single market, with the exception of some financial services, but freeing it from EU rules on agriculture, fisheries, justice and home affairs
◾ The Swiss model: Britain emulates Switzerland, which is not a member of the EU but negotiates trade treaties on a sector-by-sector basis
◾ The Turkish model: The UK could enter into a customs union with the EU, allowing access to the free market in manufactured goods but not financial services
◾The UK could seek to negotiate a comprehensive Free Trade Agreement with the EU, similar to the Swiss model but with better access for financial services and more say over how rules and standards are implemented
◾The UK could make a clean break with the EU, relying on its membership of the World Trade Organisation as a basis for trade
Many of those campaigning for exit argue that none of the country-based models would work for the UK.
They want a Free Trade Agreement which would not involve the UK accepting the supremacy of EU law, the jurisdiction of the European Court of Justice, the free movement of people or the requirement to pay significant amounts into the EU budget.
Given the fact that there is already full regulatory compliance between the UK and EU, they say it would be easier to negotiate than past trade deals.
They point to Canada, which recently signed a Comprehensive Economic and Trade Agreement with the EU, which is set to eliminate trade barriers in most areas but does not require free movement or budgetary contributions, as an example of what is possible.
Better off in: An "amicable divorce" is a pipe dream, pro-EU campaigners argue. France, Germany and other leading EU nations would never allow Britain a "pick and mix" approach to the bloc's rules. Norway and Switzerland have to abide by many EU rules without any influence over how they are formed and have to pay to access the single market. Negotiating a comprehensive free trade agreement could take years and have an uncertain outcome. And if Britain went for a completely clean break with the EU its exports would be subject to tariffs and would still have to meet EU production standards, harming the competitiveness of British business. The end result could be a trade war between Britain and the EU, some have warned, which could cripple Britain's export industries.
◾Read more: What we know so far about UK's in-out referendum
What would be the impact on British jobs?
The run-up to the EU referendum is likely to be dominated by competing claims about how many millions of jobs will be lost or gained by Britain's exit. All such claims come with a health warning. Coming up with a precise figure is difficult as there is no way of knowing if threats by foreign companies to scale back their operations in the UK would come to pass or, indeed, how many jobs would be created by the reshaped economy that might emerge in the wake of an exit.
Better off out: There would be a jobs boom as firms are freed from EU regulations and red tape, say those arguing for an exit, with small-and medium-sized companies who don't trade with the EU benefiting the most. In its recent paper, the EU Jobs Myth, the free market Institute for Economic Affairs seeks to debunk the claim that 3-4 million jobs would be lost if Britain left. "Jobs are associated with trade, not membership of a political union, and there is little evidence to suggest that trade would substantially fall between British businesses and European consumers in the event the UK was outside the EU," it argues. "The UK labour market is incredibly dynamic, and would adapt quickly to changed relationships with the EU."
Better off in: Millions of jobs would be lost as global manufacturers moved to lower-cost EU countries. Britain's large, foreign-owned car industry would be particularly at risk. "The attractiveness of the UK as a place to invest and do automotive business is clearly underpinned by the UK's influential membership of the EU," said a KPMG report on the car industry. The financial services sector, which employs about 2.1 million people in the UK, also has concerns about a British exit. "The success of the UK financial services industry is to a large extent built on EU Internal Market legislation. To abandon this for some untried, unknown and unpredictable alternative would carry very significant risks," said global law firm Clifford Chance in a report by think tank TheCityUK..
What about the impact on the economy as a whole?
Much would depend on the trade deals Britain managed to negotiate with the EU and rest of the world after its exit.
The best-case scenario, according to think tank Open Europe, is that the UK would be better off by 1.6% of GDP a year by 2030. That is assuming the UK carried out widespread deregulation after its exit and managed to strike favourable trade deals. The think tank adds: "A far more realistic range is between a 0.8% permanent loss to GDP in 2030 and a 0.6% permanent gain in GDP in 2030, in scenarios where Britain mixes policy approaches".
The Centre for Economic Performance, at the London School of Economics, says the worst-case scenario is a 6.3% to 9.5% reduction in GDP, "a loss of a similar size to that resulting from the global financial crisis of 2008/09". The best case, according to their analysis, is a loss of 2.2% of GDP, although it does not take into account as wide a range of factors as the Open Europe study.
What about immigration?
Better off out: Britain would regain full control of its borders, say anti-EU campaigners. UKIP wants to see a work permit system introduced, so that EU nationals would face the same visa restrictions as those from outside the EU, which it says would reduce population growth from current levels of 298,000 a year to about 50,000. This would create job opportunities for British workers and boost wages, as well as easing pressure on schools, hospitals and other public services.
Better off in: Britain might have to agree to allow free movement of EU migrants as the price of being allowed access to the free market. In any case, pro-EU campaigners argue, immigration from the rest of the EU has been good for Britain's economy. The UK's growth forecasts are based, in part, on continued high levels of net migration. The independent Office for Budget Responsibility says the economy relies on migrant labour and taxes paid by immigrants to keep funding public services.
Would Britain save money in membership fees?
The UK's net contribution to the EU, taking into account the rebate, was £11.3bn in 2013. That is more than four times what it was in 2008. It is about the same amount as the UK government spends on transport every year.
Better off out: The UK would save billions in membership fees, and end the "hidden tariff" paid by UK taxpayers when goods are exported to the EU, caused by red tape, waste, fraud and other factors.
Better off in: The UK's contribution to the EU budget is a drop in the ocean compared with the benefits to business of being in the single market.
What would be the effect on trade?
Better off out: The EU is not as important to British trade as it used to be, and continuing turmoil in the eurozone will make it even less so. Even if Britain did not manage to negotiate a free trade deal with the EU it would not be as disastrous as EU-enthusiasts claim, argues economist Roger Bootle in his book The Trouble with Europe: "It would place the UK in the same position as the US is currently in, along with India, China and Japan, all of which manage to export to the EU relatively easily." The UK would be free to establish bilateral trade agreements with fast-growing export markets such as China, Singapore, Brazil, Russia and India through the World Trade Organisation.
Better off in: The EU is the UK's main trading partner, worth more than £400bn a year, or 52% of the total trade in goods and services. Complete withdrawal from the EU would see trade barriers erected, with car exports to the EU, for example, facing a 15% tariff and imports a tariff of 10%. "The idea that the UK would be freer outside the EU is based on a series of misconceptions, that a medium-sized, open economy could hold sway in an increasingly fractured trading system dominated by the US, the EU and China; that the EU makes it harder for Britain to penetrate emerging markets; and that foreign capital would be more attracted to Britain's economy if it were no longer part of the single market," the pro-EU Centre for European Reform said in a recent report.
Would the UK's influence in the world change?
Better off out: The UK would remain a key part of Nato and the UN Security Council and a nuclear power, with a powerful global voice in its own right. The Eurosceptic Bruges Group wants an end to the "discredited" principle that Britain acts as a transatlantic bridge between the US and Europe, saying the country should make self-reliance its guiding principle.
Better off in: Stripped of influence in Brussels, Berlin and Paris, Britain would find itself increasingly ignored by Washington and sidelined on big transnational issues such as the environment, security and trade. America and other allies want Britain to remain in the EU. The UK risks becoming a maverick, isolated state if it leaves.
What would happen to Britons working in Europe, and EU citizens working in the UK?
Better off out: Britain would gain full control of its own borders, with migration in and out of the country regulated solely by British law. It would be more difficult for EU citizens to move to the UK, although those already living here are unlikely to be removed.
Better off in: A lot would depend on what kind of deal was reached with the other EU nations. Britons may have to apply for visas to enter EU countries and those already living there may face integration rules, such as proving they can speak the language before gaining long-term residency rights. There would also be uncertainty for many EU workers now paying taxes in the UK - what benefits, if any, would they be entitled to?
Would taxes change?
Better off out: The EU has limited power over tax, which is largely a matter for national governments. The exception is VAT, which has bands agreed at the EU level. Outside the EU, the UK would potentially have more flexibility.
Better off in: "Tax avoidance and evasion will reach crippling levels as our economy becomes increasingly wholly owned by foreign multinationals that make tax avoidance in Britain central to their business strategy," argued the pro-European Observer newspaper in an editorial.
Would Britain's legal system, democratic institutions and law-making process change?
Better off out: It would be a major shot in the arm for British democracy as the Westminster parliament regained its sovereignty and reconnected with voters. The country would be free from the European Arrest Warrant and other law and order measures.
Better off in: Britons benefit from EU employment laws and social protections, which would be stripped away. Withdrawal from the European Arrest Warrant could mean delays for the UK in extraditing suspects from other European countries; and the UK already has some opt-outs from EU labour law, including the Working Time Directive.