"On June 23, 2016, British voters are expected to answer the question 'Should the United Kingdom remain a member of the European Union?' The outcome of the referendum remains difficult to predict. Polling results have varied from week to week and continue to suggest a close race with a significant number of undecided voters. Some polls in mid-June have shown a late shift in favor of the Leave campaign."
CRS Insight, IN10449
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html