Boris Johnson has told Forces News "we have to take some brave decisions" and recognise that parts of the kit used by UK forces need to be modernised "to move forward".
However, when asked about possible cuts to troop numbers in the British Army, the Prime Minister neither confirmed nor denied reports.
He was speaking ahead of the Defence Command Paper's publication, which is expected later today.
"Modernisation in the 21st century is expensive, but we're making that investment," Mr Johnson said.
He said the increase in Armed Forces spending is aimed at modernising the defence technology used by the United Kingdom.
"What we're trying to do is to create a step-change and a hyper leap forward in terms of the technology that we use."
The Prime Minister added: "Without it, we wouldn't be able to develop the kind of kit that we're going to need in the decades ahead."
He also repeated his promise that "there are no redundancies" involved in the process.
"Individual serving men and women should be reassured there are no redundancies involved in this," Mr Johnson said.
The Prime Minister announced in November a £16.5bn increase in defence spending over the next four years, focusing on the future battlefields of space and cyber.
The Integrated Review into the future of the UK's foreign, defence and security policy was published last week.
The 100-page-plus document was billed as the most radical reassessment of Britain's place in the world since the end of the Cold War.
Cover image: Prime Minister Boris Johnson speaking to Forces News on Monday.