On 12 January 2013 a letter from Sweeney was published by the Guardian newspaper containing data relating to benefits arising from the United Kingdom's membership of the European Union, and the former European Communities, constituting an ingredient for debate over the forthcoming referendum on the nature of Britain's membership of the EU. This letter was subsequently quoted in a speech by Olli Rehn, then Vice-President of the European Commission. The letter re-surfaced during the 2016 referendum campaign on UK membership of the European Union and went viral with over 100,000 shares.