The former Labour Chancellor warned that the Better Together group, made up of different pro-Union political parties, will be heavily outspent as a result of Alex Salmond's SNP Government's access to public money.
Mr Darling said yesterday there was a "huge disparity" in the resources at his disposal compared with the Yes Scotland effort, which is backed not only by wealthy donors but by taxpayers' cash.
The main pro-UK parties are also targeting voters with their own campaigns and, in a major initiative at taxpayers' expense, the UK Government is co-ordinating action across Whitehall to present a detailed case for Scotland staying in the UK.
The Conservative-LibDem Coalition has published a number of analysis papers, paid for by taxpayers,which have highlighted what it claims are the true costs of independence.
The Scotland Office has also produced information packs for student groups, trade unions, councils and small businesses north of the Border as part of its drive to highlight the benefits of the Union.
The campaigns are allowed to spend a maximum of £1.5 million each in the final 16 weeks before the poll in a bid to ensure a level playing field, while the Scottish Government's activities will be restricted during the last month. Before then, there are no limits on campaign spending or government action.
Mr Darling said: "I'm concerned there is a huge disparity. We are getting more and more individual donations every day but we can't match having access to public funds.
"The Nationalists have a smaller number of donors but they have got more money than we have. Our funding position is improving, we are raising money.
"We will be able to start a public advertising campaign in the next month or so but we are going to get outspent on this - no doubt about that. What I find irritating is that it is such an uneven contest, where they can use the financial power of government alongside their own party stuff. Look at the White Paper. That is a manifesto more than anything else, yet public funds were taken to pay for it. It simply isn't right."
Mr Darling said he had expected senior civil servants "to come down like a ton of bricks" on moves to fund the White Paper with public money.
Producing, delivering and advertising the 650-page document has already cost £1.25m of taxpayers' cash.
The figure includes nearly £85,000 spent on billboards during a two-week spell last December.
The White Paper controversially includes SNP plans going beyond the 2016 election, which would normally feature in a party-funded manifesto.
Better Together, funded by supporters, is about to spend at least £1m on a cinema, billboard and press advertising campaign emphasising uncertainties over independence, including currency issues.Yes Scotland recently launched its own major billboard campaign, amid reports it has booked at least £1m of space.
Yes Scotland has not published details of its donations for almost a year, when it declared funding of £1.7m. However, it is thought to have ongoing support from EuroMillions winners Colin and Christine Weir.
Better Together declared donations of £1.1m last April and a further £1.6m in mid-December.
A Scottish Government spokeswoman said: "The 1997 devolution referendum shows there is a clear precedent for providing public information."
Meanwhile, a new poll by TNS put backing for a Yes vote on 28%, down by a percentage point since February.
Support for No was 42% while the don't-knows rose by a point to 30%.
The survey was conducted before an ICM poll at the weekend showed the gap between the two sides narrowing.