The Prime Minister has insisted the Chancellor guarantee defence spending until 2025 ahead of an upcoming integrated defence review, the Times reported.
The review, which was paused in April as the coronavirus crisis took hold in Britain, will 'define the Government's ambition for the UK's role in the world and the long-term strategic aims for our national security and foreign policy.'
However, Mr Sunak is said to be pursuing a one-year settlement for defence worth £1.9billion as part of a spending review set to be unveiled on November 25.
An insider explained: 'The Prime Minister and Dom [Cummings] think that the issue is pivotal to Britain's place in the world after Brexit.'
Boris Johnson has insisted the Chancellor guarantee defence spending until 2025 ahead of an upcoming integrated defence review, reports claim
However, Rishi Sunak is said to be pursuing a one-year settlement for defence worth £1.9billion as part of a spending review set to be unveiled on November 25
A second source claimed Mr Cummings, who is 'f****** furious' about the decision, is pushing for a multi-year settlement alongside Defence Secretary Ben Wallace.
Mr Johnson met with the Chancellor on Tuesday, sources said, but the pair were still 'miles apart' after they spoke.
A Whitehall insider added another meeting between the Prime Minister, Mr Sunak and Mr Wallace planned for the next day was then cancelled.
There are fears the Ministry of Defence's Integrated Review could be 'pointless' without the £15billion multi-year settlement to back it.
The department could face a 'black hole' of up to £13billion in its ten-year equipment budget if a cash injection is not imminent, it was said.
It comes as Mr Sunak is set to unveil Government spending plans for the next year on November 25, leaving the Chancellor and PM with weeks to hash out the details.
The Chancellor had already confirmed he was scrapping a planned multi-year spending review in the wake of the tumult caused by the pandemic.
It comes as Mr Sunak is set to unveil Government spending plans for the next year on November 25, leaving the Chancellor and PM with weeks to hash out the details
Instead he will set out a 12-month plan with its sights firmly set on coping with the dire financial impact of the global shutdown.
The Prime Minister had wanted to use the three-year spending review to set out his masterplan for how he intends to deliver on his promise to 'level up' the nation.
Mr Sunak said on Wednesday: 'On November 25 I will deliver the 2020 Spending Review alongside the Office for Budget Responsibility (OBR) forecast, setting out spending plans for the next year so we can continue to prioritise our response to Covid-19 and protect jobs.'
The shift to a single-year review will have a knock-on impact on the wider programme examining the UK's defence spending and foreign policy priorities.
The integrated review of security, defence, development and foreign policy is intended to define the Government's vision for the UK's role in the world over the next decade.
The Prime Minister's official spokesman said last week: 'We are considering the implications of the one-year spending review on the integrated review and we will provide an update on that in due course.'
The lack of a long-term funding settlement will make it difficult for the department to plan for the future at a time when its budget is already under massive strain.