The Nationalists insisted the "knives are out for Alistair Darling".
While one senior Labour source berated the Conservatives for helping the pro-independence cause, another was more sanguine, insisting there was not a problem with Mr Darling's position, that he was doing a fine job and he urged people to "look at the polls".
He said: "The White Paper was meant to be a game-changer for the Nats but it has had little effect; it's so full of holes, it's helped us. If you want an answer to the criticism of Alistair, look at the polls."
A poll at the weekend, carried out after the White Paper's launch, showed 27% of voters planned to vote Yes in the 2014 referendum while 56% said they would vote No; 17% were undecided.
Another snapshot, published today but carried out before the White Paper, put support for independence on 26%, support for the Union on 42% and with as many as 32% of people undecided.
With just 10 months to go before the referendum, a Conservative whispering campaign against Mr Darling is said to have begun as Tory HQ appears increasingly worried the Edinburgh MP is not making a strong enough case for the United Kingdom.
One declared: "The man has never run a campaign," adding: "He is comatose most of the time."
Another branded him "useless", noting: "He's not a very good communicator. You need someone like Gove or [Jeremy] Hunt, but it's difficult because the head of the pro-Union campaign has to be a Labour figure; it's hard for a southern English Tory to criticise him publicly."
Meanwhile, a No 10 source described Mr Darling as a "dreary figurehead" for such a crucial role.
Asked if Scot Michael Gove, the Education Secretary for England, should replace Mr Darling, David Cameron's spokesman said the Prime Minister "thinks Alistair Darling is absolutely the right person to lead the campaign and he has the highest regard for the way in which he is doing it".
One senior Labour figure brushed aside the "Tory mischief-making", insisting Mr Darling was doing a sterling job but noted that the campaign could always do better.
Another accused the Tories of "playing with dynamite", adding: "This is not some political game; this is the future of the Union."
For his part, Mr Darling has declined to comment.
While there are concerns complacency has crept into Whitehall, The Herald has been told by Labour sources that there is a danger, given the consistency of the polling, that it could do the same in Labour ranks.
There is said to be talk of former prime minister Gordon Brown being brought in to lead the No campaign.
"We have to remember [Mr Brown] is more popular in Scotland than he is in the rest of the country. But God help us if it takes Gordon to win the referendum," said one source.
Last night, the SNP seized on the criticisms of Mr Darling's leadership. "The No campaign are fighting like ferrets in a sack and the underlying problem is their total negativity about Scotland," said Stewart Maxwell, the Nationalist MSP.
"The No camp has failed to present any positive vision for Scotland and now the knives are out for Alistair Darling. But as we saw with the sacking of Michael Moore and his replacement by Alistair Carmichael, the problem for the No campaign isn't the messengers, it's the message."
He added: "Not only are people in Scotland becoming fed-up with Mr Darling's inability to offer something positive but the UK Tory Government, who are calling the shots in the No campaign, are clearly getting anxious about the prospect of a Yes vote."