Alex Salmond is facing fresh calls to agree a single question for the referendum on whether Scotland should be independent.
Former SNP politician Margo MacDonald said the First Minister should ditch any plan to ask a second question offering more devolved powers while remaining part of the Union.
Ms MacDonald, an Independent MSP, told the Scotland on Sunday newspaper: "Ditch the second question, because you can't deliver it. The only thing you can deliver is independence."
Her comments were in wider criticism of the pro-independence Yes Scotland campaign group, which she said has "no shape, no boundaries, no premise".
She said the detail of what an independent country will look like should already have been aired.
Ms MacDonald, who backs independence and believes the referendum can be won, told the paper: "I think if the SNP feel they can win the referendum without going into details, they'd prefer that. I don't know why there has always been this secretive strain that runs through them."
Opposition parties said the SNP must put forward a single Yes-No question in the referendum, due to be held in autumn 2014.
Labour external affairs spokeswoman Patricia Ferguson said: "Margo is well-respected in Nationalist circles and her intervention will come as a hammer-blow to Alex Salmond's approach.
"There is now growing consensus that there should be one, straightforward question. In that sense, Margo is merely agreeing with the SNP deputy leader, the chair of the independence campaign, and its director."
Scottish Conservative leader Ruth Davidson MSP said: "The electorate will not be fooled by yet more diversionary tactics from the Nationalists to disguise the fact they are unable to answer legitimate questions on what separation would mean for millions of Scots.
"Scotland is facing its biggest decision in 300 years and the public has a right to answers on the big questions such as currency, pensions and defence — and a single question on whether we should remain as part of the United Kingdom.
"Both the chairman and the chief executive of the separatist campaign agree the Scottish public should be given a straight choice on whether or not Scotland should remain within the United Kingdom."
The advisory board for Yes Scotland will be chaired by former MP Dennis Canavan, whose appointment was announced last week.
He will be joined by Deputy First Minister Nicola Sturgeon; Dan Macdonald, a property developer; Tasmina Ahmed-Sheikh, a solicitor and businesswoman; Elaine C Smith, the actress and comedienne; Colin Fox, Socialist politician and former MSP; Pat Kane, musician and writer; Sarah-Jane Walls, a businesswoman, and restaurateur Andrew Fairlie.
Sir George Mathewson, former chairman of the Royal Bank of Scotland, is taking up the role of honorary vice-president of Yes Scotland. Although he will not sit on the board, Sir George will be available to offer advice and guidance.
Yes Scotland was launched with a high-profile event in May, bringing together pro-independence politicians and supporters, including famous faces such as actors Brian Cox and Alan Cumming.
The opposing No campaign held its launch a few weeks later, with former Labour chancellor Alistair Darling heading up the Better Together group.
Blair Jenkins, chief executive of Yes Scotland, said: "This advisory board brings a rich diversity of experience, knowledge and skills to what will be the biggest campaign of its kind ever launched in this country.
"This is a broad church of opinions, ideas and visions, but with one shared goal — to deliver an independent Scotland.
"The advisory board will help to shape the tone and content of the Yes Scotland campaign, and make sure we present a strong and confident case for independence."
Mr Jenkins said that other board members could be added as the campaign goes on.
He hoped this would include a representative from the Scottish Green Party after its conference in October.
The party announced last month that it was delaying signing up to the Yes Scotland group until a formal decision was taken by its members.
There will also be a Yes Scotland advisory council of local and regional representatives of the campaign from all over Scotland.