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NEWCASTLE-UNDER-LYME, ENGLAND - MAY 02: British Prime Minister David Cameron launches the Conservative Party's European and local election campaign durng a speech at JCB World Logistics Centre on May 2, 2014 in Newcastle-under-Lyme, England. During his speech to workers and guest at JCB he promised to 'make sure' UK voters get a referendum on whether to leave the European Union. (Photo by Christopher Furlong/Getty Images)

NEWCASTLE-UNDER-LYME, ENGLAND - MAY 02:  British Prime Minister David Cameron launches the  Conservative Party's European and local election campaign durng a speech at JCB World Logistics Centre on May 2, 2014 in Newcastle-under-Lyme, England. During his speech to workers and guest at JCB he promised to 'make sure' UK voters get a referendum on whether to leave the European Union.  (Photo by Christopher Furlong/Getty Images)