Kemi Badenoch has called on Sir Keir Starmer and his potential Labour leadership rivals to cut welfare spending and redirect savings to defence.
The Conservative leader offered her party's support to get the necessary legislation through parliament in a letter to the prime minister, as well as to his possible successors.
It comes after John Healey and Al Carns resigned from their respective roles as defence secretary and armed forces minister over the long-delayed defence investment plan (DIP).
In her letter, Ms Badenoch told the prime minister it was "time to get serious".
She said: "We cannot have our military inadequately funded at a time of growing threats. The funding must also not be backloaded when the pressures are urgent.
"I have made several offers to work with you in the national interest to reduce benefit spending so we can invest more in our defence. Sir Tony Blair, the longest-serving Labour prime minister, has urged you to accept them," she wrote.
"Since the Parliamentary defeat of your modest attempt at welfare reform in the summer of last year, it is obvious that your left-wing MPs will not support any real attempt to cut the welfare bills. Therefore, the support of the Conservatives will be critical to delivering substantive reforms that will reduce the benefits bill."
Ms Badenoch had also sent the letter to Mr Carns, as well as Andy Burnham, Wes Streeting, Catherine West, Darren Jones and Ed Miliband.
Mr Carns has signalled he would take part in any leadership contest.
"If someone fires a starting pistol, I'm not afraid of gunfire," he told Times Radio.
Read more on Sky News:
Military chief writes to PM amid worry over defence spending plan
Defence spending plan delay has 'undermined UK's credibility'
A Downing Street spokesperson says the prime minister spoke to NATO secretary general Mark Rutte earlier on Saturday about the DIP.
"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," they said.
The prime minister also reiterated his aim to raise defence spending to three per cent of GDP in the next parliament, and that national security remains the government's top priority.
The spokesperson also confirmed Sir Keir would publish the DIP before next month's NATO summit in Ankara.
(c) Sky News 2026: Kemi Badenoch urges Labour leadership rivals to cut welfare spending and boost defence
Doctors strike in England called off after new offer from government
Derbyshire police officer investigated for using AI to 'create evidence' in multiple cases
Thousands attend anti-racism rally in Belfast after week of unrest
Counter-protesters clash with anti-racism rally in Glasgow
Three teenagers arrested on suspicion of murder after man, 21, dies in Essex park
Kevin Sinfield knighted in King's Birthday Honours - as Helen Mirren and England's Lionesses receive awards
Two teenagers arrested after girl critically injured in loading vehicle incident
Majority of young people want better online protections, according to results of major government consultation
