SCOTS who want to see a much more powerful Holyrood will not automatically vote for independence in September's referendum, according to a new analysis of constitutional preferences.

A new report by polling expert John Curtice suggests supporters of so-called devo max remain ­sceptical about the consequences of full independence.

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Analysing findings from the latest Scottish Social Attitudes survey, he argues that most devo max supporters are "unlikely" to vote Yes simply on the grounds a No result would scupper hopes of further powers for Holyrood.

The report, published yesterday, came as both sides in the debate stepped up their efforts to win over people who say they want extensive additional powers devolved to the Scottish parliament.

Labour and the Conservatives are set to unveil new policies on the constitution within weeks, which will be put to voters in the 2015 Westminster election in the event of a No vote.

Yes campaigners, however, have seized on divisions within both parties and repeated their claim that independence was the only way to guarantee more powers.

Professor Curtice, a consultant with ScotCen Social Research, the body behind the comprehensive annual attitudes survey, found devo max supporters harboured considerable doubts about independence.

Only 27%, for example, believed independence would improve the economy and only 38% felt it would strenghen Scotland's voice in the world.

He wrote: "To that extent at least most of them are unlikely to be won over to the Yes side in the referendum simply on the grounds that they have come to suspect that more devolution might not happen after all."

As in past years, devo max - devolving everything apart from defence and foreign affairs to Holyrood - emerged as the most popular constitutional arrangement.

The survey of nearly 1500 voters found 32% backed devo max; 31% independence; 25% the status quo, and 8% were opposed to any devolution. However, there was only muted enthusiasm for some of the likely consequences of devo max.

Mr Curtice said: "Both Labour and the Conservatives have to decide in the next few weeks what proposals for more devolution they wish to bring forward in advance of the independence referendum.

"Doubtless supporters of the idea will note its apparent widespread popularity, while sceptics will point to the relative lack of enthusiasm for some of its consequences.

"But given that as many as 44% of those whose first preference is devo max have yet to decide how to vote in September, working out the best way of appealing to their sometimes seemingly inconsistent feelings and aspirations could well prove vital in determining the eventual referendum outcome."

Blair Jenkins, the chief ­executive of the Yes Scotland campaign, said: "The clear conclusion is the only way for people to achieve what they want and what Scotland needs is to vote Yes."

SNP MSP Bruce Crawford said: "These figures show there is a strong majority for key areas such as pensions, welfare, taxation and the financial powers needed to transform childcare to be decided in the Scottish Parliament rather than by Westminster. The only way to gain these powers is to vote Yes."

Drew Smith, Scottish Labour's constitutional spokesman, said the party would set out plans to "strengthen" Holyrood next month.

He said: "Devolution gives us the best of both worlds - a strong Scottish Parliament backed by the economic certainty and security of the United Kingdom."

A Scottish Conservative spokesman said: "As polling shows, twice as many Scots support devolution compared with independence."