General Sir Richard Shirreff, who recently stood down as Nato's Deputy Supreme Allied Commander in Europe, also said there was "no certainty" about an independent Scotland, which would be seeking to rid itself of Trident nuclear weapons, becoming a member of Nato.
The Nationalists, who have made removal of nuclear weapons a clear red line in any post-independence talks, hit back, with defence spokesman Angus Robertson saying the people who had a "dangerous and amateurish" defence policy were those who had sent troops to an illegal war in Iraq and who were wasting tens of billions of pounds on Trident - the Coalition.
"Nato's own stated intention is for membership to be open to all European democracies that meet the membership criteria and, given that Scotland occupies a key strategic location in the North Atlantic, our continued membership will clearly be in the strong interests of the rest of the alliance," insisted Mr Robertson.
Gen Shirreff's broadside came as Labour plan to up the ante on defence.
Vernon Coaker, the Shadow Defence Secretary, will today visit Rosyth and unveil a "stark" new No campaign poster designed to puncture claims by the Yes campaign that defence jobs would remain if Scotland broke away from Britain.
With the Nato summit coming just two weeks before polling day, it is widely expected that present and former alliance chiefs will speak out against the SNP proposals to rid Scotland of nuclear weapons, as many believe such a move would remove them from the UK altogether and thus reduce Nato's nuclear capability within Europe at a time of a growing Russian threat.
Gen Shirreff poured cold water on any idea of a swift entry into Nato for an independent Scotland, noting: "It is highly unlikely Nato will agree to any further expansion while the promise of Nato membership made to Ukraine and Georgia in 2008 is still on the table."
He also noted: "It remains unclear how other members of Nato will view the disruption to the coherence of Nato defence caused by moving the submarine fleet out of Scottish waters."
In damning criticism of the SNP Government's White Paper prospectus for independence, Gen Shirreff added: "As an experienced professional soldier, nothing I have seen or heard persuades me that Scotland's safety or security would be enhanced one iota if it became a separate country.
"On the contrary, having reviewed the Scottish Government's White Paper, I find the proposals amateurish, unrealistic and lacking any clear strategic purpose."
Unionist politicians seized on his remarks.
Ruth Davidson, the Scottish Conservative leader, described them as "utterly damning" and insisted the Scottish Government's threadbare plans for a Scottish Defence Force were an insult to the professionalism of Scotland's servicemen and women.
Sir Menzies Campbell, the former Liberal Democrat leader and North East Fife MP, who is stepping down at the 2015 General Election, asked: "Why should we believe the armchair generals of the SNP on defence when those who have actually seen action are branding their White Paper proposals amateurish and dangerous?"
Stressing that one of the fundamental responsibilities of the state was to ensure the security of its citizens, he added: "This intervention from a highly respected former senior Nato commander would suggest that press releases rather than strategic priorities were foremost in the minds of the SNP when their defence policy was being drawn up."
Meanwhile, Labour sources explained that the latest campaign poster was meant to underscore the contrast between Clyde shipyards having a full workload as part of the UK, fitting out two new aircraft carriers, and the uncertain future that would await under independence.
A Better Together source said: "The last few weeks have shown the Yes campaign thinks they can get away with stating things that are not true. We felt it was time to put the choice very starkly before people.
"There are a huge number of uncertainties in defence terms, on Nato, on uncosted proposals, the nuclear deterrent and crucially the issue of jobs, both in industry and in the Armed Forces. This is about saying to people it is not long to go now; there is a need to be clear about the choice that is involved: the UK has never built a complex warship outside the UK."
He added that the poster was a "plea" from the Shadow Defence Secretary as Labour tries to return to power at Westminster in next year's General Election.
"Vernon (Coaker) wants to be the UK Defence Secretary," he said, "and he knows the UK needs Scotland and Scotland needs the UK in defence terms."
As well as shipbuilding jobs on the Clyde, thousands of people are employed in the wider defence industry in Scotland as well as on bases in Kinloss and Lossiemouth, and from Faslane to South Uist.
Mr Coaker stressed: "The jobs of thousands of Scots in our defence industry are secured by being part of the UK. They play a critical role in supporting our Armed Forces and helping to keep us all safe and secure. One thing we know beyond doubt is that leaving the UK would cost defence jobs."
The Shadow Secretary of State added: "The idea that somehow everything would carry on as it is just because Alex Salmond says so simply isn't credible."