Scotland's security against terrorist attacks would be put at risk by independence, according to the former head of GCHQ.
In a damning assessment of the Scottish government's plans for security in their white paper, David Omand said thinking about post-independence intelligence plans was filled with "optimistic assertions" and financial black holes.
"For any newly independent country security is going to be one of the first and most important duties. You can't be a little bit independent when it comes to security," Omand told the Telegraph.
"What any country is going to need in setting off down this road are time, money and goodwill. All three are going to be in pretty short supply to take the SNP white paper at face value.
"I worry that the time [set aside] is too short, the money is probably not available and - if you're not careful - goodwill isn't available either, which makes it a big ask."
Omand, who was GCHQ director for two years before becoming permanent secretary at the Home Office and then UK security and intelligence coordinator, warned that setting up a new intelligence agency would take twice as long as the white paper predicted.
The SNP has set aside just £200 million for its new security agency, dubbed McMI5 by critics.
Omand raised concerns about the SNP's insistence that it could take a portion of Britain's spying infrastructure.
"You can't divide up ten per cent of the doughnut in Cheltenham and just hand it over as a slice to a new Scottish government," he commented.
"One of the things about the white paper is that it's filled with assertions, mostly optimistic assertions, which is part of the political game. But I think after a 'yes' vote there would have to be some very hard thinking about how to make this work."
Omand also warned that the SNP had underestimated the political and financial cost of removing Trident.
Removing the nuclear deterrent would cost billions, he predicted, while also negatively affecting Scotland's relationship with the US and making it harder for it to join Nato.
Labour shadow defence minister Gemma Doyle commented: "As Sir David Omand says, the white paper is full of optimistic assertions. There is no detail about what kind of intelligence and security service a separate Scotland would have, how much this would cost and what this would mean for our security.
"There is a clear pattern emerging whenever an expert raises concerns about Alex Salmond's plan to break up the UK.
"Whether it’s trade unions, shipyard workers, supermarket bosses, large employers, the prime minister of Spain or now Sir David Omand, the response is always the same – everybody else is wrong and only Alex Salmond is right. It's just not credible."