Embattled Liz Truss was forced to insist she would hike military spending today after a Defence Minister threatened to quit.
Armed Forces Minister James Heappey warned he would resign if she ditched a Tory leadership race promise to lift the military budget to 3% of GDP by 2030.
It is currently 2.12% - only just meeting the NATO benchmark of 2%.
Asked if he would quit if there was a U-turn on defence spending, Mr Heappey said: "Yeah, but no one has said that 3% is not going to happen by 2030.”
Insisting he would step down if that changed, he added: "Yeah, we need to be spending 3% of our GDP on defence of our nation by 2030 because there is no prosperity without security."
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Mr Heappey said he and Defence Secretary Ben Wallace, who is tipped as a potential successor to Ms Truss in Downing Street, both believe the pledge "must be delivered”.
Following Mr Heappey’s intervention, No10 insisted it would meet the end-of-decade target - but threw into doubt a pledge to lift spending to 2.5% of GDP by 2026.
The PM’s spokesman said: "We are obviously committed to maintaining the UK's position at the forefront of NATO - that's why the PM committed to raise defence spending to 3% of GDP by 2030.
"The shape of that increase will be set out at future spending reviews in the normal way."
The Tories had already abandoned a key manifesto pledge on defence spending to “increase the budget by at least 0.5% above inflation every year of the new Parliament”.
