SNP plans to rid an independent Scotland of Trident nuclear weapons are incompatible with the party’s intention for the country to join Nato after a Yes vote, a former British spy chief has warned.

Sir David Omand, the former head of GCHQ, said the SNP’s policies on Nato and Trident were guilty of “magical thinking”.

Omand, a Scot who was born and raised in Glasgow, was also the UK’s first Security and Intelligence Co-ordinator, a post in which he oversaw all of the country’s spying agencies and developed CONTEST, Britain’s current counter terrorism strategy. He made his comments in a wide-ranging, exclusive interview with The Herald on Sunday.

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Omand also warned that a newly independent Scotland would struggle to set up its own functioning intelligence services, and would find itself heavily reliant on British security agencies such as MI5, MI6 and GCHQ. After independence “Scotland would be vulnerable”, he added.

Omand also said:

• A newly independent Scotland may be spied on by England if London thought Edinburgh was acting contrary to its interests.

• Russia would definitely interfere in any future independence referendum.

• Friendly nations such as America, France and Germany could also start spying on Edinburgh if a future independent Scotland gave cause for concern over security.

• The UK has not spied on the Yes movement – but would if there was any sign of subversion by a hostile foreign power like Russia.

Omand was Director of GCHQ from 1996 to 1997. In 2002 he became the UK’s first Security and Intelligence Co-ordinator. He served in this post until 2005, in which capacity he oversaw all of the UK’s intelligence agencies, and was responsible for national counter terrorism strategy and ‘homeland security’. He also served for seven years on the Joint Intelligence Committee.

On the issue of intelligence services in a future independent Scotland, Omand said: “You have to keep asking ‘are we going to be better off in defence and security terms as a result of independence’ and the answer to that is no, I can’t see how you can be better off.”

In terms of security, Omand added: “I think independence does pose a significant risk.” Managing that risk “would require significant adjustment to what I read as being the position of the Scottish National Party, namely its anti-nuclear stance, and the magical thinking about the level of security that would be enjoyed in an independent Scotland without significant assistance from England.”

Tackling threats from hostile states, international terrorism and organised crime would cost a lot, he warned – as would the establishment of an independent Scotland’s armed forces.

Scotland would depend on cooperation from London post-independence and any messy divorce proceedings could damage goodwill and collaboration, though “all logic would say that you want to continue to work as closely as possible”.

Regarding the creation of post-independence intelligence services, Omand said: “Obviously, you can’t say it’s impossible but it would be quite expensive and would require active support from England.” He added: “If you don’t have the active support of a ‘big brother’ you’re not going to get very far.”

On the risk of England spying on Scotland, he added: “Key to it would be: would there be well substantiated fears in London that Edinburgh would be taking decisions that would directly harm the security of the citizens of England – which they might not intend as harm but would be decisions which could inadvertently have knock-on effects?”

Referring to the potential for other allies spying on a new Scotland, Omand said: “Nato allies are going to be asking some very searching questions, particularly in Washington, but also in Paris and Berlin, about what is this new Scotland up to.”

Over fears of Russian cyber interference in another referendum, he said: “You can be sure there would certainly be attempts to interfere with a Scottish referendum campaign.”

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Omand insisted British intelligence has never spied on the Yes movement but said: “If there was evidence of a foreign state interfering in the referendum or trying to subvert the movement … then of course that would be a legitimate target."

The SNP’s policy on Trident, Omand said, “makes Nato membership problematic”. He suggested one way of resolving the dilemma would be “a long lease on Faslane and Coulport [by England from Scotland] and you swallow your non-nuclear instincts … I have no answers to any of these problems, all I can point out is that it’s difficult and it’s expensive and I think I and my fellow Scots deserve to have the proposition fairly set out before any talk of a further referendum.

“There’s a risk of falling into magical thinking as you can’t actually say how any of this would be done – you’re just kind of assuming that somehow it will be.”

Referencing an independent Scotland joining the EU, he added: “I’m sure there’s a majority of EU states which would say, on the security front, ‘we’d like you in Nato too’, and that comes with a price as Scotland would be expected to pay its reasonable share of defending its very critical geographical position in the north Atlantic.”