Stuart Crawford, who co-wrote a report on how Scotland could defend itself if it split from the UK, said it would cost more than £200 million a year on top of the capital costs for a Government Communications Headquarters like that in Cheltenham.
The ex-Army officer told the Commons Scottish Affairs Committee: "There's no question Scotland would be able to replicate GCHQ and all its tentacles because the budget would be prohibitive – not just the capital costs of setting up a GCHQ in Scotland but also the running costs. An independent Scotland would have to rely on intelligence being fed down from the rest of the UK."
Mr Crawford was more upbeat about the prospect of an independent Scotland attracting enough home-based intelligence specialists.
He said that although the potential pool of recruits would be smaller, the security services could offer more attractive pay and conditions as there should be enough money in the budget.
Earlier this week Deputy First Minister Nicola Sturgeon said an independent Scotland would have its own security service.