Ben Zaranko Associate Director at the Institute for Fiscal Studies, said: "If defence spending does need to rise significantly, it's difficult to overstate the seriousness of the fiscal challenge this would pose to the government.
"Meeting the pressures of an ageing population on the NHS while simultaneously ramping up defence expenditure, in an era of stagnant growth and elevated interest rates, would be an epochal challenge - and certainly not one that could be met while sticking to the letter of Labour's manifesto promises.
This government has promised to adequately fund public services, but it is also committed to keeping a lid on borrowing, imposing a fiscal rule that requires tax receipts to cover day-to-day spending.
Labour has already pledged to increase defence spending from 2.3% to 2.5% of GDP but, as of yet, there is no date for when the target will be met - unsurprisingly the Treasury wants to push it back as far as possible.
She could eat into the headroom against her fiscal target - £9.9bn - but that is already set to shrink considerably when the Office for Budget Responsibility (OBR) publishes updated economic forecasts alongside the budget next month.
