The government could have to tear up its defence strategy and start again if the United Kingdom votes to leave the European Union, according to a leading defence adviser.
The latest Strategic Defence and Security (SDSR) was carried out in 2015, five years after the previous such exercise, and played a crucial role in setting the policy priorities and budgets of the Ministry of Defence, the security services, the Foreign and Commonwealth Office and the Department for International Development.
Among other key commitments, the SDSR confirmed the government's intention to hold defence and security spending at 2% of gross domestic product, as well spending 0.7% of GDP on overseas aid.
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But a new research paper by Professor Malcolm Chalmers, the deputy director-general of the Royal United Services Institute and a former special adviser at the FCO, says that a vote for Brexit in June's referendum would mark "as significant a shift in national strategy as the country’s decision in the late 1960s to withdraw from bases East of Suez".
In such circumstances, Chalmers – who was consulted by the government as it drew up both the 2010 and 2015 defence reviews – says it would be "appropriate for the government to conduct a new Strategic Defence and Security Review, despite the fact that the last such Review was only completed in November 2015".
"This would likely be an exceptionally busy time across Whitehall, with most government departments affected in some way by the implications of a Brexit vote" – Professor Malcolm Chalmers
Chalmers says current plans for defence spending over the next decade, which the SDSR was designed to provide certainty over, might also have to be revisited if warnings about the economic toll of an exit from the EU are borne out – and says a fresh SDSR could come as early as next year.
Analysis produced by the Treasury has claimed that a British exit from the EU could hit the public finances by £36bn a year, although the Vote Leave camp has dimissed those figures as unreliable, with campaigner John Redwood saying Treasury analysis "should not be trusted".
While the RUSI deputy says the exact timing of a new SDSR would depend on the broader economic picture facing the UK in the wake of a vote to leave, he says there would be "a strong case to begin a new, post-Brexit SDSR process by the end of 2016, with completion by the spring or summer of 2017".
"This would likely be an exceptionally busy time across Whitehall, with most government departments affected in some way by the implications of a Brexit vote," Chalmers notes.
"Although the final outcome of negotiations with the EU on the terms of exit may not have been completed by the end of 2016, the government would likely have a basic idea of what the broad outcomes of the negotiations might be.
"New national policies and capabilities would be needed, most obviously in relation to trade, EU migration and the wide range of regulatory instruments that would be repatriated from Brussels.
"At the same time, if current predictions prove correct, then the first years after an exit vote could see an economic downturn, calling into question Spending Review 2015 budgetary settlements that were predicated on the assumption of healthy economic growth through to the end of the decade."
The MoD, which last year committed to a near-30% reduction in its civilian workforce, may also be forced to find "further efficiency savings" if a wider government-wide Spending Review is triggered by Brexit, Chalmers says.
"The deeper the post-referendum cuts that were necessary, the less viable [...] avoiding difficult choices would be," he adds.
Chalmers also points out that a vote to leave the European Union would mean the UK was no longer part of the bloc's common foreign and security policy. And while he says a fresh SDSR could be used by the British government "to distance itself from European security concerns" and shift ts focus towards wider global commitments, he warns that the UK could also come under "considerable pressure to retain, and perhaps even increase" its commitment to collective European defence as an ongoing member of the North Atlantic Treaty Organisation (NATO).