SERIOUS doubt has been cast by the Ministry of Defence on the SNP Government's desire that an independent Scotland, free of nuclear weapons, could ever become a member of Nato – a nuclear alliance.

The observation was made after the MoD highlighted praise this week for the UK's nuclear deterrent from Anders Fogh Rasmussen, the Nato secretary-general.

In a letter to Philip Hammond, the Defence Secretary, Mr Rasmussen noted the greatest responsibility of the Alliance was to "protect our territory and populations against attack as set out in Article 5 of the Washington Treaty".

He insisted a "great example of this has been the UK's independent strategic nuclear forces which, through their constant vigilance and professionalism over these past 45 years, have helped ensure the freedom and security of the Allies".

The secretary-general added: "These important UK capabilities will continue to play a crucial role as part of Nato's appropriate mix of nuclear and conventional forces that both deter and defend against threats to our Alliance."

With this in mind, Mr Rasmussen congratulated the UK on the successful conclusion of the 100th patrol by the Vanguard submarines under Operation Relentless.

Last night, Mr Hammond welcomed the secretary-general's acknowledgment that the UK's "ballistic missile submarine force forms a key part of the Nato security umbrella, which protects the whole Alliance".

The Secretary of State insisted that just because the Cold War was over, this did not mean Britain should weaken its deterrent. "No-one knows what threats we may face in the future: powerful nuclear arsenals remain while a number of non-nuclear states are attempting to acquire nuclear weapons," he said.

A defence source added: "Given Nato's reliance on the UK's deterrent, it's not clear it would allow membership to an independent Scotland that was against the deterrent."