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Sovereignty

On the day of the referendum Lord Ashcroft's polling team questioned 12,369 people who had completed voting.[6] This poll produced data that showed that 'Nearly half (49%) of leave voters said the biggest single reason for wanting to leave the European Union was “the principle that decisions about the UK should be taken in the UK”.

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Immigration[edit]

Lord Ashcroft's election day poll of 12,369 voters also discovered that 'One third (33%) [of leave voters] said the main reason was that leaving "offered the best chance for the UK to regain control over immigration and its own borders."'[8]

Immediately prior to the referendum data from Ipsos-Mori showed that immigration/migration was the most cited issue when Britons were asked 'What do you see as the most/other important issue facing Britain today?' with 48% of respondents mentioning it when surveyed.[9]

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Reading this from the bottom up: Immediately before the referendum 48% mentioned immigration as a major issue and in the analysis of the votes one third mentioned it as their main reason for voting as they did.   That is a very substantial figure, even if it the case that 49% gave sovereignty as the main reason. So the initial evidence would seem to suggest that the 33% could well have been the deciding factor.

It is also the case that there are social reasons people do not always give the honest answer when surveyed, so the figures could be closer than the 33-49% would suggest. There is no need to dwell on that though, since 33% is already substantial.