Cameron, who wants Britain to remain part of the 28-nation bloc, will go to Parliament Monday after a weekend that saw London Mayor Boris Johnson announce he is in favor of leaving the union.
The British pound sterling fell in early currency trading Monday on concern that the vote could be closer than expected.
Johnson wrote in The Daily Telegraph that the referendum offers a "once-in-a-lifetime" chance for real change. He says remaining in the bloc would mean more federalism and less democracy.
Johnson is joined by other Conservative Party figures although Cameron enjoys the support of key Cabinet ministers.
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