Penny Mordaunt launched a defence of the Royal Navy, warning her fellow ministers against proposed cuts.
The Commons leader, who is also an honorary captain in the Royal Navy Reserve, warned: "The Royal Navy and its partners must keep pace with the growing capabilities of other nations.
"If not, Britain's interests cannot be secured.”
This comes amid an ongoing dispute over how much money is allocated to Britain's defence.
Penny Mordaunt launched a defence of the Royal Navy, warning her fellow ministers against proposed cuts
PA
Defence Secretary Grant Shapps refused to give an exact date for when defnece spending will increase to 2.5 per cent of GDP.
He said: "There is a trajectory upwards.
"I can't give you the exact date because we've always said it's as the economic conditions allow.
"But the point is we're working to a plan.”
He claimed the UK is "comfortably" above NATO's target of spending 2 per cent of GDP on defence, adding that the UK's defence budget is second only to the United States'.
Asked about Mordaunt's comments, Shapps responded: "I'm in a post where there are a lot of people with opinions and a lot of people who've been in the military and armed forces will often express them."
Last week, Shapps announed that 20,000 troops will be deployed in Europe alongside Nato forces in an attempt to bolster global security.
He set out plans for the UK to respond to the end of the so-called "peace dividend" enjoyed by the West since the end of the Cold War.
This came just days after the Government authorised strikes on Houthi rebels in Yemen and unveiled a further £2.5bn UK support package to Ukraine.
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Shapps warned that the "foundations of the world order" are being "shaken to their core", arguing that Britain "must be prepared" to deter its enemies.
The deployment of 20,000 troops will help to support "one of the largest NATO deployments since the Cold War", Shapps said.
Some 16,000 of them will be troops from the British Army, deployed across eastern Europe from February to June 2024. They will be supported by British taking tanks, artillery, helicopters, and parachutes.