Support for independence dipped slightly at the end of last month by one point to 34% amongst those certain to vote, a TNS poll found, with more people shifting to undecided (22%) and support for the union standing still at 44% compared with the previous month.
Yes Scotland said support for Yes sits at 44% once undecideds are excluded, eight points up on last autumn and putting a Yes vote “within reach”.
But Better Together said Nationalists’ “claims of momentum are difficult to take seriously” with polling numbers unmoved since the start of the year.
TNS surveyed 1,011 Scots aged 16+ between May 21-28, largely before the Yes and No campaigns announced rival claims about Scotland’s economy and the Conservatives’ proposals for more devolution in the event of a No vote.
For the first time, TNS pressed crucial undecided voters to say which way they are likely to swing.
Nearly a fifth (19%) of undecided voters said they are likely to vote Yes against 15% who said they are likely to swing to No, while around two-thirds remained steadfastly unsure.
Better Together said the small sample of undecided voters makes it difficult to draw any conclusions from this result, but Yes Scotland said it confirms its own internal polling of undecideds which shows that “seven out of 10 have moved to Yes”.
Blair Jenkins, Yes Scotland chief executive, said: “This is a very welcome poll - of those certain to vote, excluding undecideds, Yes now sits at 44%, which is eight points up on last autumn, and within reach of the winning post in September.
“The poll also reflects our own findings about the large number of undecideds moving to Yes. Our detailed research shows that of the undecided who’ve made up their minds, seven out of 10 have moved to Yes.”
Better Together campaign director Blair McDougall said: “It is encouraging that yet another poll shows the campaign to keep Scotland in the UK with a strong lead. As we get closer to the vote people are starting to think seriously about the consequences of leaving the UK for our pensions, pound and public services in Scotland.
“With polling numbers identical to where they were at the start of the year, Nationalist claims of momentum are difficult to take seriously.”
Tom Costley, head of TNS Scotland, said “With only 100 days to go to the referendum, while many have already made up their minds and are engaged in the debate, it is evident that many people who are keen to cast their votes have not yet made up their minds.
“There is no doubt that both campaigns have the opportunity of securing additional support, depending on the messages which they communicate to these undecided voters before they have to make a decision on September 18.”
TNS also surveyed 1,090 adults in England and Wales, with 61% saying Scotland should not be a separate country, up eight percentage points since August. Support for Scottish independence has slipped from 22% to 18%.
Mr Costley added: “It seems that, while opinion in Scotland has shown little change over the last few months as the debate has continued, there is growing opposition in England and Wales to the idea of Scotland breaking away.
“It may be that, as people in the rest of the UK are exposed to the debate about Scotland’s future to a much greater extent than before, they are thinking more about the impact of independence.”