Gordon Wilson said confusion over the issue of a second question had drained public support and showed ministers believed they could not win outright.
The attack came as it emerged the pro-nationalist Scottish Independence Convention, fronted by Elaine C Smith, has called on First Minister Alex Salmond to reject a "meaningless" second question.
There were also reports that Professor Stephen Tierney, who will advise the Scottish Government on the referendum, has suggested two questions could be seen as a ploy.
Last night, Labour said Mr Salmond was looking increasingly isolated over the referendum.
Mr Wilson, who led the SNP from 1979 to 1990, warned that many within the party were perplexed by the First Minister's stance on a second question.
He accused the Scottish Government of trailing hints, adding bluntly: "There is only one reason why they would do so – defeatism, stemming from a belief that they can't deliver a Yes vote for independence."
Patricia Ferguson, Scottish Labour's constitutional spokeswoman, said: "Alex Salmond is looking increasingly isolated in his campaign for a fall-back option on the referendum as a consensus builds around a single question.
"When the academic he has hired to have oversight of the referendum tells Alex Salmond this is just a ploy, it's time he started listening and got on with asking people straight whether they want to break up Britain or not."
The Scottish Government hit back, saying ministers had always said they would take a broad range of views into account before making a decision.
Analysis is ongoing of the more than 26,000 responses to a public consultation on independence.
A second question – the so-called devo-max option – would ask about extra powers for Scotland.
A number of academics, including John Curtice from Strathclyde University, told MPs earlier this year the option attracting the broadest public support was for Holyrood to get greater power.
David Cameron has already pledged extra powers for Scotland – but only if it rejects independence in the referendum.
The Prime Minister has also refused to set out what those extra responsibilities might be, saying it is a question for Scottish politicians to decide.
However, the Tory-LibDem Coalition has also insisted if a second question is on the ballot paper it will not loan Holyrood the legal powers needed to hold such a vote.
The Herald revealed earlier this month that Coalition sources believe Mr Cameron could face a crunch point within months, where he must decide whether or not to use Westminster's power to order its own referendum.
This may happen if it is felt the Scottish Government is dragging its heels on the two questions issue.
Yesterday, Scottish Secretary Michael Moore reiterated the Coalition's accusation that the First Minister is admitting defeat by refusing to rule out including a devo-max option.
Opposition parties also pointed to an opinion poll last week that showed support for Scotland leaving the UK was at just 30%.
The Scottish Government said: "The Scottish Government's policy is independence, and the straightforward, fair question on independence is the one we have outlined in the consultation paper.
"At the same time, we recognise that there is substantial support across Scotland – from individuals and organisations – for increased responsibilities for the Scottish Parliament short of independence.
"We set out our preferred question in the consultation document but have always said we would listen to the view expressed in the responses we received."
The SNP also accused Mr Moore of offering voters a pig in a poke by refusing to set out what extra powers Scotland might get if it opposes independence.
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