The European Union would struggle to cope if the UK left, according to a senior eurozone central banker.
If the Conservatives win the 2015 general election then they have promised to hold an in/out referendum two years later on the UK's membership of the 28-state EU.
"The UK leaving would constitute an enormous shock and [be] very difficult to manage," said Benoit Coeure, a French economist who sits on the executive board of the European Central Bank (ECB), adding "we don't need that kind of shock".
Critics of the EU in Britain argue that it is undemocratic, bureaucratic and wasteful and it erodes national sovereignty.
There is a popular anti-EU sentiment across the UK, reflected by the victory in the country's 2014 European Parliament elections of Nigel Farage's eurosceptic UK Independence Party (Ukip).
But backers of the EU say it gives unfettered access to a single market of 500 million citizens, bringing a wealth of jobs and money into the UK because of its trade with other member states.
While most argue that reforms such as completing the single market by extending it to services as well as goods are necessary, they say the benefits outweigh the costs of membership.
Several business groups in the UK, such as the CBI and British Chambers of Commerce, also want to remain in the EU.
But they back reform to strip away some of the red tape for firms, such as the rigorous health and safety laws stemming from Brussels.
British Prime Minister David Cameron pledged a referendum on the EU to stave off criticism of his leadership from the back benches of his own Conservative party, where there is a vociferous eurosceptic core. He wants the UK to stay in the EU, but is pushing for reform in Brussels.