The Office for Budget Responsibility costs the path from defence spending of 2.6% of GDP in 2026 to 3.5% by 2035, the target agreed through the North Atlantic Treaty Organisation (NATO), at roughly £40bn a year of extra spending in 2025/26 prices. The Institute for Fiscal Studies (IFS), an independent research institute specialising in tax and public spending, puts about £25bn a year of that still unfunded against the plans announced so far 1.
Unfunded does not mean unaffordable. It means no revenue source or offsetting cut has been identified, so the money has to come later from tax, from borrowing, or from something else being reduced. For a sense of scale on this register's own terms, £25bn a year is roughly three-quarters of the sovereign ledger's entire interest bill for the quarter to June 2026.
What makes a commitment like this different in kind from the distress on the other ledgers is that it is voluntary and dated. A council's deficit arrives whether anyone decides anything; the 2035 defence path is a political undertaking with nine budgets left in which to fund it or quietly not. Ten years is long enough for the arithmetic to be deferred repeatedly and short enough that each deferral compounds what the later years have to carry. Watch which of the two numbers moves in the coming fiscal events. If the £25bn shrinks because money was found, the commitment is real; if it shrinks because the 3.5% date slides, it never was.
