Ed Miliband is said to be defying Sir Keir Starmer over potential cuts to his Net Zero plan to pour more funding into Britain's defence.
The Energy Secretary is said to be resisting calls to cut spending on the green agenda by at least one per cent of capital budget savings, insiders said.
The cuts would fund the long-awaited Defence Investment Plan (DIP), which Sir Keir confirmed in Prime Minister's Questions would be published before the next Nato assembly in a few weeks' time.
No10 could, in turn, force Mr Miliband's hand by cutting funding for long-term projects in the Department for Energy Security and Net Zero, including hydrogen production and heat pumps.
However, the Prime Minister is said to be keen to go further and save £9billion by scrapping carbon capture and storage, which has never been proven to be commercially viable.
One Whitehall source told The Telegraph: "While investing in sustainability is clearly important, it is also unsustainable to leave Britain’s wind farms vulnerable to Russian attack."
However, an ally of Mr Miliband said his team "does not comment on speculation".
Earlier today, the Defence Secretary resigned after declaring that too little funding was being allocated to protect Britain while international tensions escalate.
Following the shock resignation, Reform UK's deputy leader Richard Tice encouraged the Prime Minister to axe funding for "net stupid zero".
Ed Miliband is said to be resisting calls to cut funding to his department
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Commenting on Mr Healey quitting, Mr Tice said: "In effect, he has acknowledged that Keir Starmer is leaving the British people less safe.
"Much of the hot air and blather we’ve heard over the past year following the Strategic Defence Review has been for nothing.
"Healey, who is widely respected, has effectively revealed a Government that is prioritising Benefit Street over the security of the British public.
"The Defence Investment Plan could be funded in a heartbeat by scrapping net stupid zero or stopping welfare for foreign nationals."
LABOUR'S 'NET STUPID ZERO' LATEST:
John Healey unexpectedly resigned this afternoon
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"But this is a Government that would rather pay people to sit at home while lacking the political courage to make the choices needed to defend the nation."
Previously, Sir Keir is understood to have pushed for increased defence spending of about £18billion, while the Treasury pushed for a figure closer to £12billion.
While all Government departments will forced to make cuts amounting to about £6billion, it is understood No10 is looking for larger cuts from Desnz.
In a scathing resignation letter, Mr Healey criticised Sir Keir's unwillingness to commit enough resources to defence, penning: "This new era for defence required further investment through the Defence Investment Plan.
"The excellent and extensive cross-government work that completed in January - overseen by you, me and the Chancellor - confirmed the scale of the challenge and the rising demands on defence.
"Since then, you have been unable, and the Treasury has been unwilling, to commit the resources that the nation needs to defend the country at this time of rising threats."
He said that "demands on defence have increased still further".
The former Secretary of State pointed to the Iran war, threats from Russia in the High North and escalation in the Ukraine conflict.