The Conservative Party and the Labour Party have been the two biggest parties since 1922, and have supplied all Prime Ministers since 1935. Both parties changed their leader after the 2015 election. David Cameron, who had been the leader of the Conservative Party since 2005 and Prime Minister since 2010, was replaced in July 2016 by Theresa May following the referendum on the United Kingdom's membership of the European Union. Jeremy Corbyn replaced Ed Miliband as Leader of the Labour Party and Leader of the Opposition in September 2015, and was re-elected leader in September 2016.