Soldiers, sailors and airmen serving with the British armed forces may be unwilling to sign up for a new independent Scottish defence force, Defence Secretary Philip Hammond warns.
In a speech in Edinburgh, Mr Hammond will describe the SNP's plans for the defence of Scotland if it succeeds in gaining independence as a "significant gamble".
A Scottish defence force would be able to offer only a "fraction" of the opportunities available in the British forces, he will say, making it difficult to attract high-calibre recruits.
"One of the challenges any armed forces around the world will face is how to attract and retain high-quality recruits. And the key to recruitment and retention is the quality of the offer you are able to make to potential recruits," he will say.
"The British armed forces are able to attract some of the highest calibre recruits because they are able to offer some exciting and demanding career opportunities, with the chance to deploy overseas on operations and training and with the cache of being among the best and most widely respected armed forces in the world.
"The nationalists have taken for granted that soldiers currently serving in the 'Scottish' regiments would want to serve in a Scottish defence force. But who knows how many Scots would want to serve a new state as part of a Scottish defence force?
"They certainly wouldn't have a fraction of the opportunities they currently have for overseas deployment and training, nor the diversity of experience or access to the quality of kit and equipment with which they currently operate.
"It is a significant gamble to assume that troops in our UK armed forces would volunteer for a Scottish defence force. All of this adds up to a set of serious questions about the SNP's military personnel plans."
In an interview with The Daily Telegraph ahead of his speech, Mr Hammond ridiculed a SNP promise to keep 15,000 service personnel in Scotland, suggesting it was like telling recruits: "Join the Navy and see the Clyde."
He said the nationalists' plans to annex the Scottish regiments from the Army and a share of other UK defence assets lacked coherence. "Taking random units and putting them together does not make an army. Half a destroyer would be no use to anyone, neither would be one frigate," he said.