Sir Keir Starmer has criticised the prime minister for "breaking a promise" not to cut British Army troops.
During Prime Minister's Questions, the Labour leader quoted Boris Johnson from the 2019 election campaign, where he pledged to maintain the Army's size.
But Sir Keir said this week's defence review would now see numbers fall by 10,000 as part of government plans.
Mr Johnson said the government was increasing spending on defence "by the biggest amount since the Cold War".
Defence Secretary Ben Wallace confirmed on Monday that the size of the Army will be reduced to 72,500 soldiers by 2025 as part of a major overhaul of the Armed Services.
He said a move towards drones and cyber warfare would mean an "increased deployability and technological advantage" for the UK, and see a greater effect delivered by fewer people.
But the SNP said the reduction in troop numbers "undermines our Armed forces" and sees the Tories "break their election commitment".
The 2019 Conservative manifesto itself does not mention troop numbers, but pledges to "continue to exceed the Nato target of spending 2% of GDP on defence and increase the budget by at least 0.5% above inflation every year of the new Parliament".
However, during the manifesto launch in the general election campaign, Mr Johnson said that his party would "not be cutting our Armed Services in any form" if they won the vote.
Sir Keir began PMQs by asking him why he made the promise.
Mr Johnson said: "That was because what we were going to do was actually increase spending on our Armed Services by the biggest amount since the Cold War - £24bn modernising our Armed Forces with no redundancies, keeping our Army at 100,000 if you include the reserves."
But Sir Keir said the defence secretary had been "absolutely clear" in his statement this week that numbers would fall, adding: "Only this prime minister could suggest that a reduction from 82,000 to 72,000 is somehow not a cut."
'Playing with numbers'
Mr Johnson insisted his government had "kept our promise" and attacked members of Labour's frontbench for their position on renewing the UK's nuclear deterrent.
He added: "There will be no redundancies, and if you include reserves, we are even keeping the Army at 100,000.
"But on top of that we are doing what is needed to modernise our Armed Forces, taking them into the 21st century.
"We are doing all the difficult things Labour shirked during their time in office."
Sir Keir accused him of "playing with the numbers", adding: "You just cant trust the Conservatives to protect our Armed Forces."
And responding to the PM's accusation that Labour had been "consistently weak on protecting this country", the Labour leader said: "What's weaker is making a promise to our Armed Forces just before the election, then breaking it and not being prepared to admit it - not having the courage to admit it."
He called on Mr Johnson to put his defence policy to a vote in the Commons.
In response, Mr Johnson said the Tories were "pro-vax, low tax and when it comes to defence, we've got your backs".
Did the PM promise not to cut size of the Army?
Labour leader Sir Keir Starmer raised Monday's announcement of a reduction to the size of the Army, saying Boris Johnson had promised at the last election not to cut the armed forces "in any form".
He's right. In November 2019, at the launch of the Conservative manifesto - when answering questions after his speech - Mr Johnson said: "We will not be cutting our armed services in any form - we will be maintaining the size of our armed services."
But that promise wasn't in the 2019 Conservative manifesto.
On Monday, the Defence Secretary Ben Wallace announced that: "I have therefore taken the decision to reduce the size of the Army from today's current strength of 76,500 trained personnel to 72,500 by 2025".
In his reply to Sir Keir, the prime minister spoke of increased defence spending. There was a pledge in the 2019 manifesto to "increase the [defence] budget by at least 0.5% above inflation every year of the new Parliament".
The spending plan for the Ministry of Defence announced last November will achieve that on average until 2025, based on inflation forecasts from the Office for Budget Responsibility.