Pollster boss: class is the big issue which will determine referendum

"Homes in my area are mostly from housing association stock. They were built in the 1950s or 60s. It's one of those places where people live all their lives and their families live there too."

O'Connor, a welfare rights worker, plans to vote for independence this September because she believes it will make government in Scotland more accountable.

"I'm not a supporter of the SNP and I've never voted for them. But it would be much more democratic to have a government elected by five million people than by 60 million. Independence is about building a better democracy."

O'Connor's views chime with survey evidence produced by one of Britain's foremost polling companies.

In an exclusive interview with the Sunday Herald, Ipsos MORI has said that levels of affluence or deprivation in a neighbourhood provide the clearest indication of how its residents are likely to vote in the referendum. It also suggests that support for independence among those in the most deprived areas is likely to grow, while Nationalist efforts to persuade middle- and high-income Scots to vote Yes are proving ineffective.

Mark Diffley, director of Ipsos MORI Scotland, described the Scottish Government's neighbourhood deprivation index, which divides Scots into five groups according to the relative wealth of their area, as "the variable that really explains what's going on".

He added: "We find that young people are more supportive [of independence], but that [support] ebbs and flows. Older people are less supportive, but that ebbs and flows.

''When you put all the variables that we collect together - where people live, their age, their gender - then neighbourhood deprivation is generally the most important factor."

In December, Ipsos MORI published data showing that almost half of the Scots living in the most deprived areas now support independence, compared to just one-quarter of those in the richest areas.

According to Diffley, this divide has become increasingly stark since the official Yes and No campaigns launched in the summer of 2012 and may continue to grow as the referendum approaches.

He says: "The difference between the most affluent neighbourhoods and the most deprived neighbourhoods is now bigger than at any time in the last couple of years, so you can see how things appear to be developing. Given the vision set out in the Scottish Government's white paper, there's very little to suggest that trend won't continue over the coming months."

However, Diffley said a majority of middle-income voters remain sceptical of the case for independence. He added that the strength of anti-independence sentiment among the wealthiest Scots slants public opinion against a Yes vote.

"Those who live in neighbourhoods that are neither very affluent nor very deprived lean slightly more towards independence than the population as whole, but they are still opposed. The problem, though, is that because those in affluent neighbourhoods are so utterly opposed, they almost weight the population against [independence]. They make it an even tougher proposition."

David McCrone, co-director of the Institute of Governance at Edinburgh University, believes a range of factors influence class attitudes towards independence.

"Partly, it's related to the propensity of working-class voters to say they are Scottish rather than British," he says. "And we know the more Scottish you feel, the more likely you are to vote Yes. By contrast, middle-class voters may feel they have more to lose in terms of [access to] the wider UK labour market and career opportunities."

For McCrone, Ipsos MORI's findings are broadly consistent with trends dating back to the 1970s. "There is a history of working-class support for constitutional change. Compare the 1979 and 1997 devolution referendums. In 1979, people in manual jobs voted strongly in favour, whereas those in non-manual jobs voted against. By 1997, the former still applied, but this time the middle class voted Yes, so the class differential narrowed."

With just eight months to go until the referendum, and Nationalists still trailing by a significant margin, these reports could re-open the debate about Yes campaign strategy.

Left-wing independence supporters have long argued that Yes Scotland and the SNP should dedicate more of their resources to mobilising low-income Scots, a large number of whom tend not to vote in elections. The Radical Independence Campaign (RIC), in particular, has been making the case for a mass voter registration drive to encourage people in the poorest parts of the country to join the electoral roll in time for the referendum. "For us, independence is the start of a wider process of social change", Jonathon Shafi, a spokesman for RIC, said. "We focus our resources on areas with low voter turnout because we want to build a movement not only of Yes voters but of campaigners and activists too."

More moderate voices within Yes Scotland and the SNP, on the other hand, believe independence will be won by persuading middle-income Scots, who make up the largest section of the Scottish electorate, that a Yes vote won't result in any radical change to Scottish society.

Commenting, Stephen Noon, chief strategist for Yes Scotland, welcomed Ipsos MORI's research but claimed the Yes campaign had to win support from Scots across the social and economic spectrum:

He said: "Increased support among people on the lowest incomes almost certainly reflects their understanding that Westminster isn't working for the people of Scotland. However, we believe that the broad appeal of a Yes vote, of Scotland being in charge of its affairs, can and will appeal across the socio-economic and demographic groups that make up modern Scotland."

Better Together, which declined to comment on this story, will be encouraged by the strength of support wealthy and middle-class Scots continue to show for the Union. However, some in the Labour Party may have cause for concern if, as Ipsos MORI expects, levels of enthusiasm for independence go on rising in the most deprived areas.