The latest poll, published today, shows support for independence fell to 30% last month, down three points compared with February's figure.
Backing for the Union also fell in the latest TNS BMRB survey, dropping one percentage point to 51%, as the number of those unsure how to vote when the nation decides on September 18 next year rose from 15% to 19% over the month.
Although both sides in the referendum race lost support, the findings are likely to come as a bigger disappointment to the Yes camp.
Yes Scotland is understood to be planning to respond to the Better Together campaign, which yesterday revealed who was funding it in the run-up to the poll on September 18 next year.
The group's Blair Jenkins criticised Better Together, led by former Labour Chancellor Alistair Darling, for accepting large donations from benefactors not based in Scotland.