How does the UK fit into the EU's plans for closer economic and political integration?
Many British MPs want Prime Minister David Cameron to call an "in or out" referendum and opinion polls suggest many British voters are disillusioned with the EU.
BBC News asked some prominent European politicians and analysts for their views on the UK's role in the EU and whether it would make sense for the UK to leave.
Martin Schulz, president of European Parliament and German MEP
The UK should remain part of the EU, but at the same time, the UK should not preach to other EU member states from the sidelines without being fully engaged in the process.
UK membership is in the British and European interest. The single market benefits the British economy hugely and the EU remains by far the biggest destination for UK trade, accounting for almost 50% of total exports.
Nevertheless, the UK has secured opt-outs or opt-ins in areas including the euro, Schengen and justice/home affairs. This shows that the UK is not comfortable with everything the EU does and in many areas remains a reluctant partner.
As for a possible referendum on EU membership - that would be up to the British government to decide. It is nevertheless clear that the UK has to make up its mind as to whether European integration is a project it wants to be fully engaged in, and is in the British interest, or whether it should observe from the sidelines. The UK's support for deeper integration of the eurozone is welcome, but some could see it as preaching from outside.
On the EU budget, I totally disagree with the UK point of view. Like the UK, the rest of the EU wants to boost economic growth and develop a low-carbon economy. The EU budget in fact finances research and development, environmental protection, development aid and the digital economy, which are all priorities for the UK government. The EU budget is in fact an investment tool for the whole EU.
In a globalised world, the UK would risk so much going it alone. Its influence would decline and it would be separated from its closest geographic and political allies in continental Europe. We need to see a Britain engaged with its European partners.
Herve Mariton, French conservative MP
Playing with the idea of a referendum is dangerous - many people think they can speak about it, but what might happen is that the UK gets out without people realising what is happening, without all the consequences being totally analysed.
If more topics are decided on a eurozone basis, in time, the UK will have to acknowledge that it left without people taking a clear decision.
On repatriation of powers from Brussels my position is similar to the UK Government's in some respects - I do not share the same topics [with the UK], but yes some topics can be repatriated to national level. I believe in subsidiarity - things should be dealt with at the best level possible.
As far as possible, things should be done at national or local level. Take environmental issues - water policy, for example, has a big impact on the budget and here many competences should be sent back to national level. When the Rhine floods it is logical to have European intervention. But EU directives do not only deal with transnational matters - there is also flooding which has no international dimension, and then one may wonder what the EU has to do with it.
Or take the return of the wolf to the French Alps - the EU says you cannot kill more than a certain number. What on Earth has Europe to do with how French farmers have to deal with wolves in the French mountains? That is not a European issue.
I think the logic of the eurozone is federalism, we had better admit it now. That will make us organise the checks and balances necessary in any democracy. The current situation is very dangerous. I believe the UK is getting more isolated in Europe. I do not know many countries that want the EU to be just a trade zone.
Emma Bonino, vice-president of Italian Senate
The UK is not the only EU country to strive for referendums on Europe or for powers to be repatriated from Brussels. It is certainly the most vocal and, by far, the most obstinate - to the point at times of being in complete denial of its national interests. This denial may well materialise if the UK were to push the looming [EU] budget confrontation to breaking point. Do not expect other leaders to come to the rescue, in wrecking or paralysing the EU.
Some facts and figures:
- Roughly half of UK exports go to the EU
- Every UK household "earns" between £1,500 and £3,500 per annum thanks to the Single Market (SM)
- 33,000 people work for the European Commission, compared to 82,000 who work for UK Revenue and Customs
- Only 6.8% of UK primary legislation and 14.1% of secondary legislation have anything to do with implementing EU obligations - not EU diktats - agreed to, approved of and signed off by UK officials
A few arguments:
- By replacing membership with a free trade agreement would Britain be better off? Don't count on it. Any agreement would have a price: ask Norway or Switzerland about their contributions to EU cohesion funds for the privilege of accessing the SM
- When speaking to China or the US, would the UK's voice be better heard in isolation? Speaking on behalf of London while shaping decisions in Brussels definitely carries more weight
- Is UK suspicion of federalism well founded? Frankly it is hard to imagine a European superstate with a budget of just over 1% of EU countries' GDP
Britain has a lot of leverage in Europe. It is up to its leaders to decide how best to use it. For instance, is the UK not keen on some EU foreign policy priorities and on deepening the SM? Well, a British commissioner runs the EU diplomatic service and Britain certainly is not alone in wanting to preserve the SM.
Ulf Sverdrup, head of Norwegian Institute of International Affairs
A UK exit from the EU is perhaps unlikely, but even discussion of the alternatives is unfortunate, both for Europe and the UK.
For the EU, a constructive and influential UK has always been a strong asset. At its best, the UK has been able to set a course for Europe and has brought in valuable perspectives, methods and approaches.
The European debate in the UK is also potentially destructive for the UK. Rather than discussing how to ensure the UK's interests in Europe, the debate, it seems, is exaggerating the significance of formal ties to the EU.
The real lesson to be learned from, for instance, Norway as a non-member is perhaps that for a modern European country with an open economy there is no escaping the gravity of European integration.
The alternatives to formal membership for the UK are also difficult to imagine. The fundamental logic of the Norwegian model - not wanting to lose the benefits of dealing with Europe, but also knowing that a majority of the electorate is against formal EU membership - might at first sight seem enticing for many in Britain.
But the Norwegian model, shared with Iceland and Liechtenstein, is complex and costly, as well as problematic in terms of democracy and national interest. From a Norwegian standpoint the model is bearable, even if it comes at a high price in terms of democracy. But Norway is a small and rather rich country with limited ambitions to influence European policy. The Norwegian solution to Europe is not one that I would recommend for others.
Derk-Jan Eppink, conservative Belgian MEP
From the perspective of a Dutchman elected in Belgium it is very important to have the UK in the EU, because economic liberalism would be dead on the continent without the UK in it. If the UK were not there it would be dominated by German corporatism and French statism. There would be more protectionism.
I worry about developments in the eurozone, I am not sure if it is going in the right direction. Is it integration or actually schemes to move more money from north to south? I think the eurozone is still in the danger zone.
The UK is using this crisis as momentum to talk about a new relationship with the EU, to get some powers repatriated - I agree with that, because Brussels is trying do too many things at the same time. We should have a broad debate about the powers that Brussels has, what it should do.
The EU should focus on the single market, international trade, the environment and foreign policy. I am worried about a fiscal union - it will create a transfer economy in the eurozone, with one part permanently subsidising the other part, and there is not public support for that.
It is already difficult in the UK if you subsidise Scotland or Wales, but if there is no shared nationhood this support is very thin. You see that in Finland, Germany, the Netherlands - those who have to pay. They are asking serious questions about what is going to happen.
The UK is asking the difficult questions - when I ask the same questions in the European Parliament I get the same reaction. They are legitimate, correct questions. Brussels just wants to keep along the same road, and "more Europe". I do not think Britain should be a scapegoat.
Leaving the EU would not be good for the EU or the UK. The EU is by far the biggest trading partner. As for renegotiating a free trade deal with the EU - do not bet on that. If the EU is united on something, the European bureaucracy will punish the UK because it left and gave up - so it will not give a favourable trade deal to the UK.
But a more efficient relationship within the EU? It is good to start negotiating on that, then have a referendum on that settlement. Otherwise it will be very emotional, they all start screaming and there is no focus on the facts.
Hugo Brady, Irish research fellow at Centre for European Reform
The UK has a natural rebalancing role between the big countries - it avoids the rush to group-think, it is not afraid to blurt out inconvenient truths at the table, and sometimes its awkwardness is constructive. The Republic of Ireland could not do that.
Ireland is probably top of a number of countries who broadly agree with the UK on 80% of its EU agenda.
On the Anglo-Irish relationship, we were equals in a forum where we both had friends who often did not distinguish between us. The EU partners reminded us we had an awful lot in common as countries. Being equal members of the club allowed the Irish to feel on a par with Britain, not looked down on. For the first time it gave the Irish a sense that both countries were allies.
Also on a day-to-day basis we agree with the British on a lot, for example protection of the common law system. Only two other EU countries use common law - Cyprus and Malta.
Both the UK and Ireland are economically liberal. The UK is an ex-imperial power, while the Irish are more multilateral, big fans of the EU because it is a multilateral forum where the small countries' rights are overseen by the European Commission.
If the UK were to leave it would be less comfortable for Ireland. The EU would probably be less economically liberal without Britain, and a fast-developing area of EU law is judicial co-operation, but the UK and Ireland have opt-outs on that. Ireland is actually more conservative, it does not like aspects of judicial co-operation, because there is a movement towards more harmonisation of criminal and civil law.
The UK is Ireland's biggest trading partner and there is a common travel area with the UK. The Irish are very uncomfortable with UK talk of leaving. The euro is irreversible for us and it would be complicated to join a looser trading relationship with the UK.
Radek Sikorski, Polish foreign minister
We hope Britain, with its genius in creating institutions, will help us to create solid European institutions.
Poland's position on the euro is different from Britain's - we have a derogation in time, you have an opt-out. We will ratify the Fiscal Treaty and have the right to be included in those discussions, because we will one day be a member of the euro.
We would like to have Britain on board because Britain is an important country, an important guardian of liberal rules within the common market.
We would be sorry to lose you because we have been friends and allies for many years and we would like to have you contribute.
I think it is important to explain to the British people the economic advantages of the EU, but also the political ones. For example, we have just imposed the toughest ever sanctions on Iran, which are having effects. Britain alone could not do it. So the EU is a force-multiplier for all of us, to do things that we cannot do on our own.
I know for example the US tells Britain: you are more valuable to us as an ally as a member of the EU, because you can affect the decisions made in the EU. Tony Blair thought creating a European superpower was a fine goal. David Cameron has said creating Britain as a sort of offshore Switzerland would be contrary to the British national interest.
It would be much better if British politicians made a patriotic, British argument about the usefulness of the EU to Britain, because I believe your interests, your trade patterns but also your political interests, lie in Europe, and we can achieve much more together.