The question of who is entitled to vote in any particular referendum will be considered and determined in the light of the subject matter. The franchise for each referendum is commonly set out in the primary legislation which establishes that a referendum is to be held.
Under the provisions of the Parliamentary Voting System and Constituencies Bill, anyone who is registered to vote in Westminster parliamentary elections will be entitled to vote in the referendum on the voting system for the House of Commons. This means that British, Republic of Ireland and Commonwealth citizens resident in the UK and meeting all the eligibility criteria who are aged over 18 can do so. Citizens of British overseas territories, including Gibraltar, are classed as "Commonwealth citizens" under the Representation of the People Act 1983. It would therefore be possible for Gibraltarians resident in the UK and meeting all other eligibility criteria to vote. One specific eligibility criterion for Commonwealth citizens is that they have obtained any leave to enter or remain that they require in order to be in the UK.
British citizens living in Gibraltar who are registered to vote in parliamentary elections as overseas electors would also be entitled to vote in the referendum on the parliamentary voting system.