Sir Keir Starmer said he disagrees with Lord George Robertson after the former Nato chief accused the Government of “corrosive complacency” on defence spending.
Lord Robertson of Port Ellen claimed the UK is underprepared for war and ministers are unwilling to make the necessary investment.
The former Labour defence secretary warned “we cannot defend Britain with an ever-expanding welfare budget” in a speech on Tuesday.
Responding to the criticism at Prime Minister’s Questions, Sir Keir said: “Let me start by saying I respect Lord Robertson and I thank him again for carrying out the strategic (defence) review.
“My responsibility is to keep the British people safe, and that is a duty I take seriously. That is why I don’t agree with his comments.
“Last February, that was seven months after taking office, I took the decision to increase defence spending from 2.3% to 2.6%, paid for by my difficult decision on overseas aid.
“Last June, at the Nato summit, I committed to raising core defence spending to 3.5%. Last November, the budget committed record funding to defence. I reaffirm those commitments now.
“The strategic defence review, is a 10-year blueprint for national security. The defence investment plan will put that into effect. It will be published as soon as possible.”
Lord Robertson had told the Financial Times there was a gap between the Prime Minister’s rhetoric on defence and the action he had delivered, saying Sir Keir was “not willing to make the necessary investment”.
During her clash with the Prime Minister, Conservative leader Kemi Badenoch pressed him on when the defence investment plan (Dip) will be published, calling for it to come before the end of the parliamentary session.
She said: “Lord Robertson’s criticisms were of the Prime Minister, and he says Britain’s national security is in peril.
“Our armed forces are at the end of their tether waiting for this Government to fund the strategic defence review (SDR).
“There are still two weeks of the parliamentary session left so why won’t the Prime Minister publish the defence investment plan before then?”
Sir Keir did not respond to this directly, instead saying the Government is “spending £270 billion over this Parliament,that’s £5 billion more this year, with defence funding increasing every year”.
The 10-year plan was due to be published in autumn last year to deliver the SDR’s plans.
Mrs Badenoch continued: “It is being reported that the Treasury is asking the MoD (Ministry of Defence) to make £3.5 billion worth of cuts this year.
“He won’t fund our military because he wants to fund more welfare.
“That’s why he has a welfare plan to 2031, but no defence investment plan at all.
“Now the Chagos surrender deal is dead.
“Will the Prime Minister put the billions saved from ditching Chagos into defence, or is that going into welfare as well?”
Sir Keir replied: “We are spending more on defence, record amounts on defence.”
He added: “We got the biggest boost of defence spending since the Cold War.
“We’ve also got the biggest pay rise for our armed forces for over 20 years.
“We’ve also got the biggest investment in military housing for over half a century.”
Mrs Badenoch had earlier taken aim at Labour MP Samantha Niblett (South Derbyshire), who said she hopes to bring sex toys into Parliament as part of her campaign for more open, inclusive and lifelong sex education.
The Tory leader said: “This is a moment of profound national seriousness, and what are they doing? They’re promoting sex toys in Parliament.
“It gives a whole new meaning to fiddling while Rome burns.”
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