Mr Sillars, who served as Mr Salmond’s deputy in the early 1990s, warned of the “extreme danger” of the move, which he claimed would bolster support for the alternative at the expense of outright independence.
Meanwhile, former SNP leader Gordon Wilson warned that Scottish troops could still be dragged into “aggressive” military overseas conflicts involving the United Kingdom if voters backed devo-plus.
The option would see Holyrood control most taxes, including income and corporation tax, as well as Scotland’s geographic share of oil revenue, but Westminster would keep control of defence.
The interventions from the two senior SNP figures came after The Scotsman revealed that the Nationalists are willing to settle for devo-plus, which falls significantly short of full fiscal autonomy for Holyrood, if voters reject outright independence in the referendum, which the Scottish Government wants to hold in autumn 2014.
The newly launched devo-plus campaign has been supported by senior figures from opposition parties such as former Holyrood presiding officer Alex Fergusson, and former Scottish Liberal Democrat leader Tavish Scott.
Mr Sillars also claimed that a vote for the extra powers could simply be ignored by Westminster unless the UK government signed an agreement to devolve the responsibilities ahead of the referendum. He said: “If the SNP is going to put devo-plus on the ballot paper, then it would have to get a signed agreement from David Cameron that the UK government would implement it. If there is not signed agreement, then it’s not worth the paper it’s written on.”
Mr Sillars said that having the devo-plus option on the referendum ballot paper would “cut down the case for independence” and dismissed the move as “nonsense”, claiming that it would introduce “ambiguity” in the vote.
He said: “This is a very weak policy for independence. The moment the SNP seems to back anything except independence, it rings the death bell for independence.
“The moment you put up something that’s different to independence and give it implicit endorsement, then you cut down the case for independence.
“You don’t persuade someone of independence by saying there’s another alternative that I quite like. It introduces ambiguity. There’s an extreme danger in shilly shallying like this.”
Meanwhile, Mr Wilson said that there would have to be a veto over Scottish involvement in “foreign military adventures” – such as those in Iraq and Afghanistan – for Holyrood if voters backed the devo-plus option. He said: “I’m worried that Scottish servicemen could get dragged into wars with the loss of life in these conflicts. This is something that [the SNP government] has to consider.”
A spokesman for the First Minister said that the Scottish Government was currently holding a consultation to see “whether support emerges for a ‘more powers’ option in the referendum”.
He said: “It is up to the advocates of devo-plus to define it – we support independence, and independence is the only constitutional option which gives Scotland the power to prevent us being dragged into illegal wars such as Iraq against our will.
“Whether the referendum contains a ‘more powers’ option or not, we are extremely confident of winning a Yes vote for independence.”