General Sir Richard Shirreff, who has just stepped down as Nato's deputy supreme allied commander Europe, said the SNP's plan is "amateurish" and that Scotland's future in Nato is "uncertain" if it leaves the UK and expels nuclear weapons from the Clyde.
His comments, in a letter sent to Sunday newspapers, were echoed by former Nato secretary general Lord Robertson, who said and anti-nuclear Scotland "would have the door slammed in its face" by Nato.
But the SNP described current UK defence policy as "dangerous and amateurish", saying it has wasted billions of pounds on nuclear weapons while sending under-equipped troops to fight "an illegal war".
Sir Richard, who has family links with East Lothian, said: "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. There is no mention of any naval aviation (yet Scotland would need a primarily naval force), no mention of air-to-air refuelling capability, no Mountain Rescue and no Search & Rescue capability. The White Paper proposals are dangerous and would leave Scotland, the UK and Nato weakened and less capable of dealing with the threats of today and tomorrow."
He added: "Any country applying to join the alliance will require unanimous agreement from all 28 current members of Nato. Just one vote against and it will not happen.
"Added to which, it is highly unlikely that 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. Given the current crisis in Ukraine, there will be no quick fixes and there can be no certainty about Scottish membership of Nato.
"As for the nuclear issue, Nato is a nuclear-armed alliance and all Nato states must accept the principle of nuclear deterrence and being part of the Nato nuclear command and control system.
"Whilst the SNP may accept the principle of nuclear deterrence, 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."
Lord Robertson, who served as secretary general of Nato from 1999-2004, told the Scottish Mail on Sunday: "Nato is getting clearer and clearer that an anti-nuclear Scottish state expelling the UK deterrent would have the door slammed in its face.
"Membership applications need the agreement of every country in Nato and the full resolution of all neighbouring disputes. The expulsion of Trident would be a real block for Scotland."
Angus Robertson, the SNP's defence spokesman, said: "A dangerous and amateurish defence policy would be one that sent troops into an illegal war without proper equipment, while wasting tens of billions of pounds on Trident nuclear weapons that can never be used - that's the UK."
Commenting on behalf of Better Together, Scottish Conservative leader Ruth Davidson said: "The first duty of government is the defence of the nation. On this evidence, the SNP has failed the test.
"General Shirreff makes it clear there is no certainty around an independent Scotland's membership of Nato. Worse, he concludes that the SNP's plans for a Scottish Defence Force are 'amateurish' and 'unrealistic'. For our servicemen and women to be faced with this prospect is an insult to their professionalism."