Uncertainties surrounding the outcome of the independence referendum remain despite polls which show a consistent lead for No, according to Westminster advisers.

A comprehensive analysis of polling data on Scottish independence has been compiled by the House of Lords library.

Recent polls "appear to suggest that, following a period of stability, there may have been a gradual tightening of the referendum race over the past six to nine months", the analysis found.

Further devolution with additional tax powers has consistently been Scotland's most popular choice since 1999 at up to 50%, with independence at about 30% and the status quo or fewer powers very unpopular at about 10%.

Only 30% believe more devolution will be delivered if Scotland votes No despite pledges from all three main parties to devolve more tax powers.

But this may not be enough to convince pro-devolution No voters to swing to Yes, with most still opting to remain in the UK even if more devolution was not on offer, the librarians found.

The Lords Library compiled data from Ipsos MORI, ICM, Panelbase, Survation, TNS-BMRB and YouGov going back as far as 1979, as well as data from the Scottish Social Attitudes Survey and advice from leading psephologists.

It concluded: "In spite of what polling and survey data can tell us, there are many uncertainties surrounding the outcome of the independence referendum.

"Ultimately, pollsters and campaigners alike will have to wait until September 18 to see how the Scottish public will vote."

Only one of the 56 polls analysed showed a majority for Yes, by a single percentage point, but the librarians highlighted criticism which pointed out that it was commissioned by the SNP and contained preceding independence questions which may have had "a skewing effect".

However, librarians also highlighted "inconsistencies" in the polls which put support for Yes between 40-45%, leaving psephologists "unsure whether No still enjoy a relatively comfortable lead or whether the Yes side are at least within sight of the winning post".

The report said: "Predicting the outcome of the independence referendum is proving something of a challenge for pollsters."

The librarians also assessed whether "doubts around the further devolution of powers to Scotland in the event of a No vote" could influence the result.

"Whilst a steady majority of respondents (around 60%) would like to see further devolution, there is some skepticism that this will actually materialise," the report said.

"Indeed, little more than a third of respondents appear to believe this would occur."

However, this skepticism does not appear to point to less committed No voters switching to Yes, the report said.

"If anything it seems that over the course of the referendum campaign an increasing share of No voters have concluded that, if faced with this hypothetical scenario (of no more devolution), they would still vote against independence," it said.

"Whilst this suggests the No campaign has not been unduly damaged by skepticism surrounding promises of further devolution, there might still be cause for concern for supporters of Better Together.

"Professor John Curtice of the University of Strathclyde warns: 'The 10% who say they would vote Yes and the additional percent who say they do not know what they would do could still be more than the No side could afford to lose should the referendum race get much tighter."

Blair Jenkins, chief executive of Yes Scotland, said: "The empty promises by the anti-independence parties of more powers for the Scottish Parliament is evidence that the ground is shifting beneath them.

"They know they are losing the argument and that is the only reason they are now talking about more powers.

"But these figures show that only a third of people in Scotland trust them to deliver - and in any event what is being proposed falls far short of what Scotland needs.

"What we need are powers over job creation, welfare and defence so that we can build a more prosperous, fairer country which is free of nuclear weapons.

"The Westminster parties are now attacking each other's proposals - if the No campaign parties don't believe each other, why should the people of Scotland?

"I believe that as people focus more and more on this debate, they will conclude that all the decisions about Scotland need to be made by people living here, because we are best-placed to make them."