Deep cuts in Government spending on tackling illegal migration, organised crime and diplomacy could have "considerable" implications for national security, a respected defence thinktank has warned.
Despite ministers' promises to protect the defence and aid budgets in the new spending round, Chancellor George Osborne's demand for the Home Office and Foreign Office to find savings of up to 40% has cast doubt on the "credibility and coherence" of Government defence and security policy, said the Royal United Services Institute.
In a briefing, Rusi said that the departments and agencies dealing with illegal migration, organised crime and diplomacy were the "missing links" in financing for the upcoming strategic defence and security review (SDSR). Additional spending totalling £400 million by 2019/20 would be needed to protect these areas, the thinktank said.
Mr Osborne wrote to all departments in July telling them to model two scenarios for this autumn's spending review, setting out how 25% or 40% of real-terms savings could be achieved by 2019/20 in a bid to find an additional £20 billion savings in public spending.
Along with the NHS and schools, defence and aid were protected from the reductions, in line with Government commitments to continue to spend 2% of GDP on the military and 0.7% on overseas assistance.
In the new briefing, Rusi research director Professor Malcolm Chalmers said that the Government had made commitments that the UK "is willing to devote the resources necessary to remain a serious power on the international stage, backing up its ambitions with real increases in funding for both defence and development".
But he added: "If the SDSR were to be accompanied by steep reductions in spending on the diplomatic network, or by significant cuts in the resources available for combating organised crime and illegal migration, it could risk undermining the wider coherence and credibility of the review.
"The Government would be open to the criticism that it was prepared to devote substantial resources to meeting international norms for defence and aid spending, while cutting spending in areas more directly related to national security, foreign policy and prosperity objectives."
Prof Chalmers said the Foreign Office had already seen a 19% real-terms fall in core spending, adding: "If the core departmental budget was to be subjected to cuts comparable to the 25% and 40%scenarios currently being discussed with the Treasury, the consequences could well dominate perceptions of the 2015 SDSR, just as cuts in defence dominated coverage of the 2010 review."
A Treasury spokesman said: "The Government is clear that security comes first - the economic security of a country that lives within its means and the national security of a Britain that defends itself and its values.
"That is why the Chancellor committed additional resources in the summer budget to the defence and security of the realm, committing to meet the Nato pledge to spend 2% of our national income on defence not just this year, but every year of this decade.
"We also recognise that the threats we face do not distinguish between different Whitehall budgets, which is why the summer budget reaffirmed our commitment to an aid budget which is creating a safer and more prosperous world, alongside a new commitment to increase the defence budget every year in real terms, and on top of that, creating a joint security fund of £1.5 billion a year by the end of the Parliament.
"Decisions on how this additional funding will be allocated will be made at the Strategic Defence and Security Review later this year, and through the forthcoming Spending Review we will set out our plan for ensuring frontline services across government are not only more efficient but better prepared to tackle challenges such as terrorism and organised crime at home and abroad."