Scottish independence is an "unknown proposition" and may require two referendums to ratify, according to a leading constitutional expert.
Alan Trench, honorary senior research fellow at the constitution unit at University College London, said a referendum without prior negotiation on Scotland's future powers would not be "fair".
He has also called for a single yes-no question to be drafted in consultation with the Electoral Commission.
On the need for two ballots, he said: "Any referendum held before independence negotiations have taken place means that voters will be voting on an essentially unknown proposition.
"They will not, and cannot, know what choices may be made about the nature of an independent Scotland in those negotiations. That creates a serious flaw in the process.
"This could be avoided by holding a referendum after independence negotiations have taken place. However, it is not clear what the mandate for holding such negotiations would be. This is a powerful argument for the 'two referendums' approach."
He added: "The common wish of the two governments to have a 'decisive' referendum militates against having one that is also 'fair'."
Mr Trench, who is frequently consulted by politicians for his constitutional expertise, has delivered his comments to Holyrood and Westminster as a single response to the two referendum consultations currently under way.
He has also criticised the "uncertainty" over timing. He said: "I see no reason why a referendum should be held sooner than the autumn of 2014, in accordance with the Scottish Government's proposal. What is important is that the timescale for a referendum be made clear as soon as possible."
Mr Trench said he is "attracted" to a second question on greater devolution - commonly known as devo-plus or devo-max - but said in practice this would be unworkable.