Since Labour is the biggest party in the group, with former Chancellor Alistair Darling as its leader, many in the party will have hoped the STUC would put its weight behind the campaign against independence.
But a paper to be published this week, A Just Scotland, is critical of both sides of the Yes-No divide and calls for "more information and less sloganeering" in order "to persuade us that social justice is more achievable as a consequence of their chosen constitutional option".
The STUC "has not reached a point where it is able to definitively recommend a Yes or No answer", it states. "It was never imagined that at this stage it would."
But the paper speaks of a challenge for the Better Together campaign "and specifically for the Scottish Labour Party", with a strong hint of concern over recent suggestions that some universal benefits should be cut.
Criticising all three parties in the campaign, it states: "There was concern and, on occasion, outright anger at some of the economic, social and international policies which have been pursued by Government, particularly at the UK level. 'Not being the Tories' and negative messages about the SNP will not suffice."
STUC general secretary Grahame Smith said: "Over the past few months we have invited members as well as those from wider communities to join the discussion on social justice and Scotland's future.
"This interim report poses many questions, reflecting a general view that the quality of the debate must improve dramatically and it must take place in a far less febrile atmosphere."
He added: "A Just Scotland lays out challenges for both sides of the debate. In particular it criticises the use of misleading figures in the debate over Scotland's fiscal position.
"The report identifies deep problems with the economic and fiscal model imagined by the leading voices in the Yes campaign. However, it also calls on the Better Together parties to outline a practical vision of how social and economic justice can achieved within the Union and to calls for detailed attention to be paid to proposals for enhanced devolution."
A spokesman for Better Together said: "We respect the decision of the STUC leadership to not adopt a formal position on the referendum at this early stage of the campaign.
"We will continue to press home our message with the leadership and also with the tens of thousands of men and women who make up the unions over the next two years. We will be doing everything we can to convince them we are better together as part of the United Kingdom."
A spokesman for Yes Scotland said: "It is encouraging the STUC recognises many of their members already intend voting Yes to independence while many more remain open to persuasion. Yes Scotland is confident that, when the benefits of independence are fully spelled out, trade union members will join the majority of their fellow Scots in voting Yes in the referendum."
SNP MSP Christina McKelvie said: "This is an important contribution from the STUC that looks at the kind of Scotland we want to see and examines the powers needed to get there.
"It will make uncomfortable reading for the anti-independence campaign, given their failure to set out any kind of positive vision for the future of Scotland. Unsurprisingly, Johann Lamont's Cuts Commission has also clearly provoked a strongly negative reaction from STUC members."