With barely a year until the referendum, a YouGov survey found support for a Yes vote at just 29%, compared to 59% for a No, the largest gap between the two sides since Alex Salmond revealed the official question in January. Only 10% of respondents were undecided.
The two-to-one lead for the No side suggests a hardening of pro-Union support, and a decline in the number of undecided voters on whom the Yes campaign have been pinning their hopes.
The new poll brings the average of all the polls to have asked voters "Should Scotland be an independent country?" since February to Yes 33%, No 51% and Don't Know 16%.
YouGov conducted the survey of 1171 Scottish adults between August 19 and 22.
It was commissioned by Devo Plus, a cross-party pro-Union group that campaigns for greater tax and spending powers for Holyrood.
The findings come as Nationalist MSPs return to Holyrood this week amid increasing scepticism that Yes Scotland and its chief executive Blair Jenkins can turn around stubbornly poor polls.
The group, which is SNP-dominated but includes the Scottish Greens and Scottish Socialists, plans to step up a gear around September 18, as the referendum campaign enters its final year.
One of those close to the group said there would be an autumn "relaunch", with greater prominence given to the minor parties.
One Nationalist said: "People at all levels in the SNP think the party hierarchy has to get a grip of Yes Scotland by the scruff of the neck."
However, one SNP MSP said if Salmond and others did weigh in it would only "piss off" the non-SNP participants in Yes Scotland.
The poll found most people - including 58% of Labour and 65% of SNP voters - wanted the anti-independence parties to set out a vision of more powers for Holyrood before the referendum. If that vision was devo plus, in which Holyrood would raise between 50 and 100% of the money spent in Scotland, 16% of undecided and Yes voters would be more likely to vote No.
Ben Thomson, chair of Devo Plus, said the poll confirmed widespread public support for an agreement on more powers for Holyrood.
Blair McDougall, campaign director for the pro-Union Better Together group, said: "Yes Scotland are further behind than they were when they launched their campaign. This poll will surely add to the sense of crisis in the nationalist camp."
A spokesman for Yes Scotland said: 'What this poll underlines is that only a Yes vote will deliver the powers that people in Scotland want their parliament in Edinburgh to have."