An in or out referendum would be legitimate.  Parliamentary and logistic precedents indicate that holding a referendum about an issue of sovereignty is practical and practicable. There have been two successfully-held national referendums, one the direct antecedent of this one (the 1975 EEC vote) and one recently (the 2011 AV vote). The history of national referendums therefore indicates that they are both constitutional and logistically viable. Furthermore, the referendum is legitimized by public desire. A BBC poll in 2009 indicated that 71% would vote 'no' to EU membership if a referendum was held, a level of dissatisfaction which indicates the need for a public discussion and vote.1  1 MORGAN, CHARLES. January 2, 2009. "No change in British public opinion on Euro" Cafebabel