UK Prime Minister David Cameron says Britain will hold a referendum on whether to leave the European Union if his party wins the next election.
In a long-awaited speech in central London on Wednesday, he said the terms of Britain's membership of the bloc should be revised and citizens should have a say.
He acknowledged public disillusionment with the EU is "at an all-time high".
The next general election is expected in 2015.
Cameron said: "It will be an in-out referendum."
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The possibility of a referendum is expected to rattle business leaders and frustrate other EU member states currently focused on stemming the euro zone debt crisis.
Already, speculation over a vote on leaving the EU has prompted a chorus of concern from around the world, stressing the importance of the UK's presence in the bloc and warning about the economic consequences of a British exit.
Even the US, which normally stays out of disputes among EU states, waded into the debate.
The White House said last week President Barack Obama told Cameron in a phone call that "the United States values a strong UK in a strong European Union".
Much of the criticism directed at Cameron has accused him of trying an "a la carte" approach to membership in the bloc.
He insisted on Wednesday that a "one size fits all" approach to the EU was misguided.
"Let us not be misled by the fallacy that a deep and workable single market requires everything to be harmonised, to hanker after some unattainable and infinitely level playing field," he said.
"Countries are different. They make different choices. We cannot harmonise everything."
