Vice-Admiral Sir Jeremy Blackham said enemies would be prepared to call the UK’s bluff, assessing that the nuclear option would be a “disproportionate” response to a conventional assault and would not be used.
He said the nuclear deterrent was “not a substitute for conventional capabilities” and claimed the costly process of replacing the submarines carrying the Trident system could pose a “severe challenge to the shrinking UK defence industry”.
In an assessment for the UK National Defence Association (UKNDA), Sir Jeremy said: “It has been UK policy that nuclear weapons would never be used against non-nuclear states party to the non-proliferation treaty, although during the Cold War first use was not actually ruled out.
“But the cardinal point is that the nuclear deterrent is not a substitute for conventional capabilities. The credibility of flexible response depends upon deferring any decision to use nuclear weapons until the very existence of the nation is at stake.
“This requirement means that conventional forces must always be of sufficient capability to deal with any lesser threat; and that one’s potential enemy must believe this to be so. “
The former deputy chief of defence staff added: “If the conventional means at our disposal are weak, the point of transition to nuclear use may be lowered to levels at which the risk of nuclear obliteration is self-evidently disproportionate to the issue at stake.
“At that point, it is likely that deterrence through the threat of nuclear use becomes incredible and can be so perceived by an opponent – a bluff waiting to be called.
“Thus, through conventional weakness, the nuclear deterrent is compromised, whether it is a rogue state or a major power that is involved.
“To be credible, the nuclear deterrent must be underpinned by strong conventional deterrence. The idea that nuclear deterrence is synonymous with strong defence is to assume ‘big bang’ is ‘big defence’. It isn’t; quite the reverse.”
He said the 2010 Strategic Defence and Security Review had created “some highly significant capability gaps” which “together with depleted equipment numbers and reduced manpower, have unbalanced our force structure, and for which no solutions are yet fully identified, let alone funded”.