ice-Admiral Sir Jeremy Blackham said cuts may leave a gap where future enemies could call Britain’s bluff, judging the UK would be unwilling to ever use a “disproportionate” nuclear response to a conventional attack.
He said the nuclear deterrent was “not a substitute for conventional capabilities”, which had been badly hit by recent Government cost-cutting.
He also warned the costly process of replacing the submarines carrying the Trident deterrent 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.”
Vice-Admiral Blackham warned the Government’s 2010 cost-cutting Strategic Defence and Security Review (SDSR) had created “some highly significant capability gaps”.
These gaps, “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”.
The Armed Forces had been given highly advanced ships and fighters in recent years, he said, but there were too few of them.
He said the Navy’s new Type 45 destroyers were “astonishing” but added “there are however just six of them”. The Typhoon fighter was “a quantum jump in capability over its predecessors; but only some 30 could be sustained in a prolonged conflict.”
He said replacing Britain’s nuclear deterrent “is likely to pose a far more severe challenge to the shrinking UK defence industry than did either Polaris or Trident”.
He warned the long-term costs and timescales had yet to be spelt out.
“So without new money, the risks to the remaining conventional programme appear to be considerable. Conventional force levels are again at risk and so therefore is the credibility both of the nuclear deterrent and of deterrence more generally.”
Sir Jeremy said conventional forces were an effective deterrent because the threat of their use was “genuinely credible”.
“Any potential adversary is likely to believe in its use but only provided that it is also clearly sufficient for the particular purpose or operation to hand. And in so doing it can snuff out dangers before they escalate.
“That is the key point of conventional deterrence – to prevent bad things happening and getting worse so that escalation towards 'nuclear territory' does not occur.”
A spokeswoman for the Ministry of Defence said: “Tough decisions had to be taken to tackle the multi-billion pound blackhole in the defence budget but it now means our Armed Forces, although smaller, will be fully funded, well trained and properly equipped to face the threats of the future.
“We remain committed to maintaining a continuous submarine-based nuclear deterrent which is the ultimate safeguard of our national security.”