Defence Secretary George Robertson denounced the Scottish Nationalists as "naïve", while the Lib Dems labelled their policy a "shambles".

But the author of the academic article - Stuart Crawford, a tank squadron commander during the 1991 Gulf War and now the SNP's deputy defence spokesman - tried to play down his ideas.

In his paper, Some Thoughts on an Independent Scottish Defence Force, which Mr Crawford wrote under an assumed name, he suggested ballistic missiles have a "number of attributes".

George Robertson and Colin Campbell clash on BBC Radio 4's Today programme

These included "reasonable cost, which make them very attractive to the armed forces of smaller nations".

He added: "They might well provide an inexpensive alternative to more costly and vulnerable weapons."

George Robertson: Labelled the SNP "naïve"

Coming as opinion polls continue to suggest the SNP could beat Labour in the forthcoming election for the Scottish Parliament, the defence secretary seized on the recent appointment of Mr Crawford as a nationalist spokesman.

He told the BBC: "The reason I take it very seriously is that we have elections coming up. This is a party that aspires to leading the Scottish Parliament and even taking the nation into independence, which is at best shows dangerous naivety on a subject of huge importance."

Liberal Democrat defence spokesman Menzies Campbell added: "To call the SNP's defence policy a shambles is an under-statement.

"Every time it is exposed to scrutiny, another wheel falls off the policy."

But Mr Crawford denied he held the view floated in the paper.

He issued a statement, saying: "I wish to put on record that I have never at any time advocated the possession by an independent Scotland of nuclear, chemical or biological weapons.

"I am certain that these are not options for an independent Scotland which it would be either sensible or necessary to pursue.

"Therefore I am totally supportive of SNP defence policy which repudiates utterly the possession of nuclear, chemical or biological weapons."

The SNP's main defence spokesman, Colin Campbell, backed this view and refused to dismiss his newly-appointed deputy.