Scottish voters want an early say in the future of the constitution, rather than stick to First Minister Alex Salmond's preferred timetable, a new poll suggests.

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 a survey Ipsos MORI 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%, up three points from a poll in August,.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.

"There does however appear to be growing support for the referendum to be held sooner rather than later which could put some pressure on the Scottish Government to alter its preference for holding the ballot in the second half of the current term."

People aged under 34 are most likely to back Scottish independence, Ipsos MORI said. Support was at 47% among those aged 18-24 and 58% for 25-34 year olds. Three-fifths (61%) who would vote SNP would also vote for independence.

The First Minister welcomed the poll, which was based on 1,001 people. Mr Salmond said: "The people of Scotland are light years ahead of the Westminster parties and the inadequate provisions of the Scotland Bill. They want real economic and financial powers for the Scottish Parliament so that we can strengthen recovery and boost employment.

"The SNP Government have always said that we are willing to have a devo-max option in the referendum, and the question for Labour and the Lib Dems is whether they wish it to be included. The only alternative for Labour and the Lib Dems is joining with the Tories in rejecting any additional powers for Scotland in the referendum, a disastrous position in Scottish politics.

"The SNP received an overwhelming mandate from the people of Scotland in May to hold a referendum in the second half of this parliament and that is exactly what we will do."

Liberal Democrat leader Willie Rennie MSP said: "This poll shows that two-thirds of Scots support the Liberal Democrat home rule policy. Home rule would deliver greater powers for the Scottish Parliament and is something that we have advocated for generations. It would mean Scotland being less dependent on a grant from Westminster and would be responsible for raising much more of what it spends. "Scotland would have more control over its destiny on the home agenda whilst sharing risk and opportunity with the rest of the UK family. "Whilst it is really concerning that independence is a real possibility now, we are pleased that support for our position is strong."