He said “we already have extra money in our budget this year” for defence, and ministers were continuing to announce contracts which give “our fighting forces the kit and equipment they need to deter aggression”.
In a directly political intervention, Lord Robertson – who is now a key government adviser – will warn in his speech: “We cannot defend Britain with an ever-expanding welfare budget.”
Speaking in Salisbury, he will say: “We are underprepared. We are underinsured. We are under attack. We are not safe… Britain’s national security and safety is in peril.”
He will add: “There is a corrosive complacency today in Britain’s political leadership. Lip service is paid to the risks, the threats, the bright red signals of danger – but even a promised national conversation about defence can’t be started.”
The prime minister’s official spokesman said: “I completely reject that, our armed forces are working around the world every minute of the day to keep us safe at home and strong abroad”.
Defence spending last year was 2.3% of GDP (around £66bn). A defence official highlighted the government’s target to spend 3% of GDP on defence by the end of the next Parliament and 3.5% of GDP on core defence by 2035.
Welfare spending is forecast to be 10.6% of GDP, or £322.6bn, in 2025 to 2026.
The SDR was published in June 2025 and the government accepted it would implement all 62 of the review’s recommendations, but is yet to share how it will fund these plans.
A government spokesperson said the spending review was “backed by the largest sustained increase in defence spending since the Cold War, with a total of over £270 billion being invested across this Parliament”.
There have been reports that the Defence Investment Plan, which will detail how priorities set out in the SDR will be paid for, has been held up over disagreements within government about new funding, as well as how to fund existing defence arrangements.
Earlier on Tuesday, Downing Street rejected Lord Robertson’s criticism and said the report would be published “as soon as possible”.
The prime minister previously said the investment plan was on his desk and was being “finalised”.
Strategic goals listed in the SDR included a Nato-first defence policy and a move to “war-fighting readiness” to establish a more lethal “integrated force” equipped for the future.
Some of the details also included a £11bn annual budget for front-line kit and the creation of a “new hybrid Royal Navy” that uses aircraft, drones, warships, submarines to patrol the north Atlantic “and beyond”.