Margaret Curran, the Shadow Scottish Secretary, said she had learned through ministerial written answers that the Scottish Government has not had a single conversation on the consequences of independence with any Whitehall department.
In a speech today to a conference in London on independence, she will accuse the SNP of giving the impression they are losing heart on their key policy and looking to having a second devo-max question on the referendum ballot paper simply to "save Alex Salmond's face".
Ms Curran told The Herald: "In less than two years, the SNP want to break up the UK but they haven't done the slightest bit of homework to find out what this will mean for people in Scotland. Their policies are based entirely on assertion and fantasy.
"The Scottish people are going to make the biggest decision they have ever faced and we have a right to get answers to the most basic of questions on separation, including what will happen to our pensions, border controls or employment law."
Today at the Scottish Politics Explained conference, the Glasgow East MP is due to say: "The SNP used to believe in independence but as every day of the campaign goes on it looks like they no longer do."
She will claim Mr Salmond's "spin and speculation" are more evidence he "knows he's lost the independence question and that's why he wants a second question in spite of his party, his deputy, the chair and director of his independence campaign".
She will add: "We are not having a referendum to save Alex Salmond's face. We are having one to decide whether or not Scotland is part of the UK. We need to settle this now in a clear, simple question: are we in or are we out?"
Last week, The Herald reported how senior Coalition figures were becoming convinced the First Minister was minded to have a second question on devo-max.
Stewart Hosie, the SNP's Treasury spokesman, did little to disabuse people that the Nationalist leadership was moving towards the two-question option.
He said: "The case for independence trumps any case for further devolution but there are those who believe in further devolution and, of course, they should be allowed to test that against independence and the status quo." Mr Salmond himself said that while the UK Government was opposed to a question on devo-max his position was "a bit different from that".
Yesterday, Downing Street said: "A single question is the right way forward. It will provide a legal, decisive referendum, which gives clarity on the issue."
The First Minister has spurned Scottish Secretary Michael Moore's call for talks on the referendum. It is thought he will respond formally in the autumn when the responses to the SNP Government's consultation on the referendum are published.
Some at Westminster believe he will use the results to argue a second question on devo-max should be included in the poll.
A spokesman for Bruce Crawford, the Scottish Government's Secretary for Parliamentary Business, said: "This is a ridiculous intervention from Margaret Curran but sadly in line with her party's thinking on Scotland.
"The fact that she wants the Conservative-led Treasury to have a say on Scotland's finances post-independence says it all and simply underlines how out of touch Labour are by backing the Tory-led anti-independence campaign to the hilt.
"Planning for independence is well under way and the structure of the state will be fully outlined in the Scottish Government's White Paper published next year."