Legal provisions for optional referendums at national level

Antigua and Barbuda
TheReferendum Act 1997
http://aceproject.org/ero-en/regions/americas/AG/antigua-and-barbuda-the-referendum-act
The Referendum Act 1997
Article 2. For the purposes of determining any question relating to the Constitution of Antigua and Barbuda or determining any other question the Prime Minister may consider necessary or desirable, there shall be held a referendum in accordance with the provisions of this Act