LONDON, UNITED KINGDOM - MAY 30: Prime Minister David Cameron makes a joint appearance with Mayor of London Sadiq Khan as they launch the Britain Stronger in Europe guarantee card at Roehampton University on May 20, 2016 in London, United Kingdom. The "guarantee card" lists five pledges should Britain remain in the EU, including the protection of workers' rights, full access to the single market and stability for Britain. U.K voters go to the polls on June 23 to vote in a referendum on the continued membership of the UK in the European Union. (Photo by Yui Mok - WPA Pool/Getty Images)
