UK is a member of the Venice commission. The Venice commission's Code on referenda states (section 3.3 - Funding, Paragraph 24): "In the event of a failure to abide by the statutory requirements, for instance if the cap on spending is exceeded by a significant margin, the vote must be annulled."
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Leave.EU have been found to have overspent spending limits by at least 10%. This is a clear violation of the Venice Commission and as a result, the Referendum result should be annulled and a new vote called.