The European Union Referendum Act 2015
1 The referendum
(1) A referendum is to be held on whether the United Kingdom should remain a member of the European Union.
(2) The Secretary of State must, by regulations, appoint the day on which the referendum is to be held....
(4) The question that is to appear on the ballot papers is-
"Should the United Kingdom remain a member of the European Union or leave the European Union?"