Nearly three-quarters of Scots want more political powers and half think they will come without independence, a major study has found.

Unionist pledges of more devolution if Scotland votes No "could pay off" in keeping the UK together, the survey of around 4,000 Scottish voters found.

A majority of voters want the most important powers to rest with the Scottish Parliament in all policy domains except defence, the British Election Study (BES) said.

A belief that more powers will be delivered is crucial in people's decision to vote Yes or No on September 18, it found.

But the SNP "might be excused in thinking that the commitment to devolution amongst the major parties is as much a matter of electoral expediency as a matter of principle", according to BES co-director Professor Ed Fieldhouse.

He said: "Wanting more devolution is not sufficient on its own to achieve support for independence. It has to be accompanied by a belief that it will not be achieved in the Union.

"What really matters is not just the level of support for more devolution, but the extent to which supporters of further powers think their aspirations will be met within the Union.

"Half of voters, we find, think that more devolution will happen even if Scotland votes No."

BES is funded by the Economic and Social Research Council and run by a consortium of Manchester, Oxford and Nottingham universities.

It conducted an internet panel survey of over 4,000 Scottish voters between February 20 and March 9 this year.

The Liberal Democrats and the Conservatives have pledged to devolve all income tax and give Holyrood power to raise 40% or 50% of spending respectively, while Labour is also offering 40% tax-raising powers with three-quarter control of income tax.

Mr Fieldhouse added: "Almost three-quarters of Scottish voters want more devolution. Moreover, even amongst those intending to vote No a majority (57%) say they want some or many more powers for Scotland compared to 96% of Yes voters.

"Defence was the only policy domain that a majority of Scottish voters thought should be reserved for the UK Parliament.

"Overall, half of voters think that devolution will happen even if Scotland votes No.

"Crucially the largest group is those that want more devolution but think it will happen anyway (41%). Of these, only 35% say they will vote Yes in September.

"Contrast this with the 71% of those wanting more devolution but not expecting it who intend to vote Yes.

"In other words wanting devolution is not sufficient for support for independence. It has to be accompanied by a belief that this will not be achieved in the Union.

"The Yes Scotland campaign therefore has two related problems. One is that not enough voters are convinced that their demands will be unmet within the Union. The other is that even amongst those who do not expect the Union to satisfy their wishes, a substantial minority say they vote No anyway or are undecided."