Labour leader Sir Keir Starmer has warned there is "a very real risk" the Armed Forces will be "stripped back even further".
Speaking in the House of Commons after the publication of the long-awaited Integrated Review, Sir Keir said this was the "chance to turn a corner" after "a decade of neglect".
He added: "This review won't end the era of retreat, in fact, it will extend it."
The review, described by Downing Street as the biggest review of UK foreign, defence security and development policy since the end of the Cold War, was published on Tuesday after delays due to COVID-19.
It sets out a number of plans for future policies, including increasing the UK's number of nuclear warheads and a "tilt" in strategic direction and diplomacy towards the Indo-Pacific region.
Prime Minister Boris Johnson said the UK must be "match-fit for a more competitive world" and in response to Sir Keir, called the Labour party "weak on defence".
Mr Johnson told MPs: "Anybody listening to (Sir Keir) would not realise we are the second biggest international donor of aid in the G7."
He added that it is "preposterous to hear the Labour leader calling for more investment in our Armed Forces when this is the biggest investment in our Armed Forces since the Cold War".
The Government announced in November an extra £16.5bn for defence over the next four years.
Mr Johnson continued: "It is ridiculous for (Sir Keir) to talk about our nuclear defences when the reality is that Labour is all over the place."
Sir Keir earlier told MPs that the review is "built on foundations that have been weakened over the last decade".
He said: "The Prime Minister has spoken about an era of retreat – he's right.
"In the last decade of Conservative government, defence spending and pay for the Armed Forces both fell in real terms. Our Armed Forces numbers have been cut by 45,000."
Sir Keir welcomed the "long overdue" increase in capital funding, the creation of a counter-terrorism operations centre and new investment in cyber, but added: "The Prime Minister can't avoid the question that everyone in our Armed Forces and their families will be asking today: will there be further cuts to the strength of our Army and our Armed Forces?
"The British Army is already 6,000 below the minimal level set out in the last review, it's been cut every year for the last decade, and it's been reported that the Army will see a further reduction of 10,000 alongside fewer tanks, fewer jets for the RAF and fewer frigates for the Royal Navy."
In response to a question from Labour's Nick Smith, MP for Blaenau Gwent, Mr Johnson said "there'll be no redundancies across the Armed Forces".
The Integrated Review warns there is a "realistic possibility" of both a terrorist group launching a successful chemical, biological, radiological or nuclear attack by 2030 and another pandemic.
It also analyses the threat from Russia and the risks posed by increased competition between states, including a more assertive China.
It states the UK will remain committed to NATO and the so-called Five Eyes security alliance.
Further details on how the review will impact the Armed Forces are due to be published on Monday.
For more discussion on the Integrated Review, click through to this week’s episode of the Sitrep Podcast.
Cover image: Royal Navy.