Plans to remove Trident from an independent Scotland would "add a dangerous period of destabilisation" to the UK's nuclear defence posture, while a Yes vote would be "deeply worrying" for the country's allies in Scandinavia and the Baltic, a former defence chief has warned.

Admiral Sir Mark Stanhope has written an open letter to Alex Salmond restating his concerns about independence in relation to the country's defences.

Sir Mark said he is "very unhappy" with the First Minister's response to a previous intervention from a number of senior defence figures in April.

He wrote: "With the referendum now imminent, I am writing to you to make clear what we believe would be the grave defence consequences of a Yes vote not just for the United Kingdom but for the people of Scotland themselves."

Sir Mark, First Sea Lord and Chief of Naval Staff from 2009 to 2013, highlighted the recent warning from President Vladimir Putin concerning Russia remaining a nuclear state.

"Your plans for the removal of all nuclear submarines from Faslane in the event of Scottish independence would add a dangerous period of destabilisation in our nuclear defence posture at a time when the international picture is clearly deteriorating," he wrote.

"On top of this, Scottish independence and its exit from Nato would be deeply worrying and unpopular with many of Scotland's natural allies, particularly our friends in Scandinavia and the Baltic who depend on the United Kingdom for mutual defence of air space and territorial waters. Scotland has contributed so much to our collective security over the years that it would be tragic if it now came to be regarded as the weakest link in our defence shield."

Sir Mark said independence would also threaten the careers of servicemen and women, while the defence industry has also issued a warning over employment.

The Scottish Government's White Paper sets out five defence priorities for an independent Scotland, including a £2.5 billion budget for defence and security, the fastest safe withdrawal of nuclear weapons, a focus on maritime capabilities, moves to build a total of 15,000 regular and 5,000 reserve personnel over 10 years, and the reconfiguration of the defence estate.

SNP defence spokesman Angus Robertson said: "This letter quite clearly sets out the difference between the Yes campaign and the No campaign. While the anti-independence camp is determined to waste over £100 billion on unusable and obscene Trident nuclear weapons, a Yes vote will rid Scotland of weapons of mass destruction.

"It is demonstrably wrong of Sir Mark Stanhope to suggest that Scotland will not be a part of Nato. The UK's most recent ambassador to Nato, Dame Mariot Leslie, made it abundantly clear on the basis of her vast experience and knowledge in this area that an independent Scotland will be welcomed into the Nato alliance - and that she will be voting Yes next week.

"An independent Scotland will work closely with our friends in partners in Nato, and play a full part in contributing to our common security."

Scottish Labour MP Gemma Doyle said: "SNP's defence and security policies have previously been branded amateurish and unrealistic by the experts, because they fall apart under any scrutiny.

"Admiral Sir Mark Stanhope is a hugely respected voice. When he speaks on defence issues you take notice, and if he says the consequences of separation could be grave, not just for Scotland but the United Kingdom and Europe, you take it seriously.

"Our armed forces and security services are world leading. They keep us safe. They are too important to risk on the assertions and dodgy sums of the nationalists.

"We do not have to risk our safety or security. We can have what the majority of Scots want - that means more powers for Scotland without taking on all of the risks of independence. It's the best of both worlds. We should say No Thanks to separation on September 18."