Ex-Defense Secretary Liam Fox on Monday joined growing calls for a national vote - a policy once advocated on Britain's political fringes, but rapidly gathering mainstream appeal.
Fox, a member of Cameron's Conservative Party, says that with the 17 eurozone members developing closer fiscal and political ties, others in the 27-nation EU should carefully consider their future.
Cameron has acknowledged Britain will likely need a new national vote on ties to Europe, but insists that shouldn't happen while the debt crisis unfolds.
Britain last held a vote on its membership of the EU, then the European Economic Community, in 1975.

