A former defence chief has warned reported plans to reduce the Army by 10,000 troops could put the UK at risk.
The changes are to be set out in a Defence Command Paper being published on Monday, with the military braced to lose some "industrial age" capabilities.
This is expected to see older tanks, ships and warplanes axed or phased out early, while the Army may see its total number of personnel reduced to 72,500.
Lord Richards of Herstmonceux, chief of defence staff from 2010 to 2013, told Times Radio he supported plans to modernise the military but added it "can't be at the expense of conventional capabilities".
"You've got to have some of the more traditional capability in case mass becomes an asymmetric attraction to one's potential opponents," he said.
"If all we've got is high-tech stuff, and they've got half a million troops that can come across the border at you, then these high-tech capabilities aren't going to be much good," he said.
"If your opponent senses that they are at a disadvantage, or their own capability is being neutered by one's own possession of those capabilities, they will look for another way of achieving their goals, and that could suddenly become numbers again, mass.
"And we certainly won't have it."
However, Defence Secretary Ben Wallace told Forces News last week that while there will be a decrease in troop numbers in some areas, there will also be an increase due to existing capabilities being retired.
"Fundamentally, the Armed Forces we're going to have is going to reflect the threat we face and what we need to do to deter that," he said.
Mr Wallace added that he is able to make the decisions "in a rising tide", with the defence budget increasing by an extra £24bn.
"These are changes to our Armed Forces that are not driven by a Treasury cut," he said.
"They are driven by what I think and my military advisors… think is the right thing to do to make sure we are ready for tomorrow's wars."
It comes after the publication of the Integrated Review – billed as the most radical reassessment of Britain's place in the world since the end of the Cold War.
As a result of the review, there will be more money for new capabilities, such as electronic warfare and the development of autonomous systems.
Labour has criticised the review, with shadow defence secretary John Healey MP saying there was a "gulf between the Government's ambitions and its actions, which is set to grow with this new review".
He added: "Further Army cuts could seriously limit our forces' capacity simultaneously to deploy overseas, support allies and maintain strong national defences and resilience."
Labour leader Sir Keir Starmer also warned last week the review wouldn't "end the era of retreat, in fact, it will extend it".
The Integrated Review also committed to increasing the number of nuclear warheads to 260, reversing a previous move to reduce the stockpile to 180.