Cameron said in a BBC interview he was entitled to block efforts by the eurozone nations to modify a treaty and create such a union unless the other nations agree to Britain's demands for a looser relationship with the EU.
"They are changing the nature of the organization to which we belong," Cameron said. "And thus we are perfectly entitled – and not just entitled but actually enabled, because they need changes – to ask for some changes ourselves."
Cameron did not suggest Britain leave the EU altogether but he said the union was undergoing "massive change being driven by the existence of the euro" and he was obligated to ensure his country had flexibility to protect its interests, The Guardian said.
Cameron said his Conservative party were expected to push for "real changes" in its relationship with the EU, which he said would be a lot easier to pursue if his party wins big in the next election. "I think we have done a good job over Europe in the last 2 1/2 years, but a Conservative government would obviously be able to go further"