Defense is a prime candidate for cuts as the new Conservative-Liberal Democrat government searches for savings to reduce a budget deficit of around 11 percent of economic output.

"Given the difficulties involved in making sharp and rapid reductions in procurement spending, the personnel budget will have to bear a significant share of (the budget) cut," Professor Malcolm Chalmers of the Royal United Services Institute said.

Britain's defense budget is already under pressure because of expensive weapons programs and the cost of keeping 9,500 troops in Afghanistan.

Chalmers had previously predicted the defense budget would fall in real terms by 12 percent by 2016, but said he had revised this to 15 percent by 2014 in light of Finance Minister George Osborne's emergency budget on Tuesday.

Osborne confirmed most of the burden of reducing the public debt would be borne by spending cuts and reiterated previous commitments to protect health and overseas aid spending.

All non-protected departments have to find savings of 25 percent in real terms over the next four years. Details of cuts will be set out in a spending review on October 20.

While defense spending is not ring-fenced, Osborne said not all departments would receive the same settlement, and acknowledged the "particular pressures" facing the education and defense budgets.

"This may be an indication that these two areas will be asked to accept smaller reductions than other unprotected departments," said Chalmers.

"Even so, given the high proportion of unprotected expenditure that they consume, neither education nor defense will be able to avoid severe reductions over the next four years."

Chalmers estimated personnel numbers would need to fall by about 15 percent, to 240,000 from 283,000, to meet the necessary reduction in spending. If spread proportionately this would mean a 30,000 cut in military personnel and a reduction in civilian personnel numbers of around 13,000, he said.