Support for independence has increased but still falls short of a majority, according to a poll.
The Ipsos MORI survey suggests 38% of people would vote to take Scotland out of the UK, up three points from a poll in August.
Voters appear to want an early say in the future of the constitution, rather than stick to First Minister Alex Salmond's preferred timetable.
The SNP leader has said the referendum is likely to take place towards the end of this five-year parliamentary session, meaning it could be held in or after 2014.
But 33% of people in the survey want it held as soon as possible, while a further 31% want it within the next two years.
Two questions could feature on the ballot, offering yes-no options on independence or for enhanced powers and responsibility for almost all taxes while remaining in the UK.
Among those saying they are certain to vote, 68% back the second option, known as "devo-max", up one point from August, 28% do not back it and 4% are unsure.
Support for independence is at 38%, while 57% disagree and 5% are still unsure.
The poll follows survey results last week which showed the SNP is twice as popular as Labour.
Mark Diffley, research director at Ipsos MORI, said: "Although our poll last week revealed growing support for the SNP, a clear majority of Scots do not currently support full independence and would prefer to remain part of the UK, albeit with the Scottish Parliament given substantial new powers."