Support for independence stands at 35 per cent, according to the Panelbase poll, eight points behind those who intend to vote No.
A similar Panelbase poll in September put Yes at 37 per cent and No and 47 per cent – a margin of ten points.
Yes Scotland chief executive Blair Jenkins said the results show the outcome is far from decided ahead of the historic vote on 18 September next year.
He said: “This survey confirms what our own research is telling us – that momentum is very much with Yes and that there is everything to play for.”
The poll also showed that large number of voters remained undecided, according to pro-independence campaigners.
A pro-Union Better Together spokesperson said: “We are taking nothing for granted and will fight for every single vote between now and the referendum.”