The first minister has been warned that he has "no mandate" from the Scottish people to put a second question in a future referendum on independence.
Alex Salmond has hinted he may offer a choice on the ballot paper for enhanced devolution - dubbed "devo max".
However, Scottish Labour leader hopeful Tom Harris said voters at the May election only gave the SNP a mandate to hold an independence referendum.
The Glasgow South MP said he did not support enhanced devolution.
Mr Harris suggested that the UK government may have to step in and create new legislation which ensured an unambiguous referendum question was asked.
He insisted that voters had given the SNP a "mandate not a blank cheque" at the Holyrood poll.
At the SNP's four-day conference a future independence referendum featured in several speeches to delegates.
Debate was sparked around the prospect that the ballot paper, which will be issued towards the end of the five-year parliament, could have two questions.
One would be a straight "yes" or "no" to independence.
The other would be on the so-called "devo max" question which would focus on full tax powers for Holyrood, with defence and foreign affairs staying under Westminster control.
Mr Harris is one of three candidates running to be leader of the Labour Party in Scotland.
The other two contenders are MSPs Ken Macintosh and Johann Lamont.