Plans for a single security and intelligence service in an independent Scotland will not offer the level of protection and support currently provided as part of the UK, according to a former chief of MI6.

Sir John Scarlett said British intelligence services have been built up over decades - "work which cannot be replicated in just a few years".

The Scottish Government set out proposals to create a single agency for security and intelligence to ensure Scotland's national security in its white paper on independence.

Writing in the Times newspaper, Sir John, who served as chief of the British Secret Intelligence Service from 2004 to 2009, said: "In my view, the Scottish Government proposals will not offer the level of protection and support currently provided by the highly-sophisticated British security and intelligence agencies."

His comments follow an intervention from Sir David Omand, the former head of GCHQ, who described the SNP's defence plans as "fundamentally flawed" and issued a warning over the future of cyber security.

Former Nato commander General Sir Richard Shirreff branded the SNP's defence policies as "amateurish" and "dangerous".

Shadow defence minister Gemma Doyle said: "Yet again, we see the experts saying one thing and Alex Salmond another.

"The SNP's defence and security policies fall apart under scrutiny. It makes no sense for us to pay more money for less security.

"The nationalists cannot tell us how much it would cost to set up new defence and security agencies, but it's clear from experts like Sir John Scarlett that we could not replicate what we enjoy now.

"As part of the UK, we benefit from a defence and security budget of £34 billion. Why give that up?"