CROSS-PARTY campaigners against Scottish independence will receives a major boost today with a poll revealing that, in the past year, a one-point lead for the Yes camp has turned into a 20-point lead for the No camp.

The six-monthly survey by TNS BMRB – taken after both the Yes Scotland and Better Together campaign launches –puts those against independence on 50% and those in favour on 30% when a straightforward yes/no question was asked.

This latter figure is the lowest received for independence in five years of polling on the subject by the Edinburgh-based pollster.

With regards to its second question incorporating a devopmax option, TNS BMRB notes that if those backing the status quo and those in favour of more powers are added together, then "support for remaining within the UK stands at 66%", compared to 23% in favour of independence.

The poll findings chime with a growing confidence in the No camp. A senior Coalition source told The Herald last week: "To be honest, at this stage we would have to try very hard to lose it. We would have to get complacent, and we are not going to."

TNS BMRB began its surveys in August 2007 when the respective figures for those against and for independence were 50% and 35%. Since then, there has been a general narrowing of the gap.

In August 2011, the Yes camp nudged ahead of the No camp for the first time with a one-point lead. However, anti-independence forces have been making ground, and the gap now stands at a substantial 20 points.

But there is another important aspect to the snapshot – the rise in support for the extra powers option.

On the second, three-option question asked by TNS BMRB, both the pro and anti-independence causes show falling support while that for more powers is rising.

During the past six months, support for the pro-independence option has fallen from 26% to 23%, while support for the anti-independence option has dropped from 32% to 29%. In contrast, support for the more powers option has jumped from 30% to 37%.

It is also interesting to note that when the third option is included, the number of "don't knows" falls markedly; in this case, from 20% to 11%, which means the pool of "persuadables" is significantly reduced.

TNS BMRB points out that about 25% of those currently supporting independence, and almost 40% of "don't knows" on the yes/no scenario, would switch to devo-max if given the option.

What the polling breakdown also shows is that the Yes campaign is failing to convince Scottish women of its argument. Only 19% of women compared to 27% of men back independence, while 39% of women and 35% of men support more powers. Some 29% of both sexes support the status quo.

Chris Eynon, head of TNS BMRB Scotland, said the anti-independence camp would be buoyed by these latest figures.

"Of particular importance is the fact that this is not simply a one-off uplift in response to launch publicity, but is the continuation of the trend detected back in January," he said.

"The balance of public opinion has swung strongly in favour of the Union and the independence movement would appear for now to have their work cut out to reverse this.

"They at least have time on their side to do so."