Cameron wants to reform Britain's EU ties and is due to set out his demands in more detail on Tuesday.
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"You can boil down all of my negotiations to one word: flexibility. Is this organisation flexible enough to make sure that countries inside the eurozone can grow and succeed, and countries outside the eurozone like Britain can find what they need as well," Cameron told an audience of business leaders.
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"If it's flexible enough, we'll stay. If it's not flexible enough we'll have to ask ourselves a very profound question: is this organisation for us?"
Cameron's speech was briefly interrupted by two hecklers who unfurled a banner reading "CBI - Voice of Brussels" in what appeared to be a protest against the employer group's position on EU membership.
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