The Army faces further manpower reductions after 2020 to make way for Britain's growing Navy, Liam Fox has warned.
The former defence secretary, who oversaw the 2010 strategic defence and security review (SDSR), said more tough decisions would be faced in the Ministry of Defence soon.
Fox said "different priorities" could result in the need for changes when the next SDSR, now just two years away, takes place in 2015.
"It's not simply a question of numbers, it's a question of where your strength goes," he told Politics.co.uk.
"For example, I think we'll need to see growth in the manpower available to our surface fleet as we start to increase the size of that again.
"Therefore balancing that with a reduction in the size of the army is something that makes sense."
Britain's military is currently engaging in a strategic 'shift to contingency', in which a period of intense, long-term deployments to countries like Iraq and Afghanistan comes to end.
There are no plans at present to increase the number of Royal Navy surface ships but defence experts say the introduction of the Queen Elizabeth aircraft carrier – and possible decision not to mothball the second Prince Of Wales carrier – would increase the numbers required.
"You could see the Army under more threat partly because it's easier to reconstitute Army personnel," Michael Codner of the Royal United Services Institute think-tank said.
"You can rebuild an infantry regiment over five years – what you can't do is build a new ship over five years."
Fox said he accepted further Army cuts posed "challenges" of its own in terms of the numbers of regulars and reserves.
Under plans announced in July, current defence secretary Philip Hammond announced a plan to reduce the number of regulars from 100,000 to 80,000. Reservists will be increased from around 25,000 to 30,000 by 2018.
Codner said the UK government faced a dilemma in its next defence review about the ways in which the military could be used to advance Britain's goal of remaining a significant player on the international stage
"It's putting troops on the ground that really brings hard influence – certainly that's the case when there is a crisis," he added.
"In the buildup it's another matter, surface ships can be used for gunboat diplomacy."
Fox said in an interview with Politics.co.uk he believed the government had to pursue an active approach to world affairs.
"It boils down to a simply binary choice: we're either going to shape the world we're in or we're going to be shaped by the world we're in," he said. That's it."
The traditional view of the UK as a country with a seat at the top table has been called into question by the Commons' decision to vote against British participation in a military intervention in Syria.
The prospect of a strike against Bashar al-Assad's regime has faded in recent days but Fox remains critical of Cameron's snap decision to rule out British participation outright.
"I'm not sure the message the Commons wanted to send was that the rest of the international community might want to act together but that Britain wouldn't," he said.