Labour leadership hopeful Andy Burnham has already said he would reduce the welfare bill to fund defence spending.
New Defence Secretary Dan Jarvis has vowed to get the armed forces the funding they need after his predecessor quit over the issue.
John Healey and armed forces minister Al Carns resigned over the long-delayed defence investment plan (Dip) on Thursday.
Mr Jarvis told The Sunday Telegraph he had a “big responsibility” towards soldiers who risk their lives for the country.
The timing of the resignations of Mr Healey and Mr Carns, along with two ministerial aides, came at a moment of peril for Sir Keir Starmer.
Mr Jarvis, himself a former soldier, told the newspaper: “The defence of our nation is a shared endeavour… I have a big responsibility in that regard now, but so do all of those people who expose themselves to risk tonight, tomorrow, next week, and we owe them a debt of gratitude.
“I have a responsibility now to them to make sure that they get what they need, and people should be very clear about my determination to fulfil those duties, to make sure that they do (get) precisely what they need.”
Mr Carns signalled he would take part in any leadership contest on Saturday, telling Times Radio: “If someone fires a starting pistol, I’m not afraid of gunfire.”
Andy Burnham, who hopes to return to Westminster in the Makerfield by-election and has made no secret of his Labour leadership ambitions, has already said he would not be “squeamish” about reducing the welfare bill to fund defence spending.
The Greater Manchester mayor said “the world has changed” and it was “obvious” the Government would have to adjust its assumptions about defence spending in response.
Meanwhile, Conservative leader Kemi Badenoch has written to potential Labour leadership rivals, calling on them to cut welfare spending and redirect the savings to defence.
Mrs Badenoch offered Sir Keir, as well as his possible successors, her party’s support to get the necessary legislation through Parliament.
The Prime Minister is likely to face difficult questions about defence spending at the G7 when members met in France on Monday.
He spoke to Nato secretary general Mark Rutte about the Dip on Saturday morning.
A Downing Street spokesperson said: “The Nato secretary general welcomed the UK’s increased investment in defence as an important contribution to the alliance and to meeting the threats we face.”
