A smaller British armed forces would mean the UK could no longer be a full military partner to the United States, former US defence secretary Robert Gates says.
Gates told the BBC that cuts in the number of military staff would limit the UK's global position.
The British government is planning major cuts to the Armed Forces.
The army is being cut from 102,000 to 82,000 over a number of years, with the 20,000 posts expected to be gone by 2020.
Navy numbers are expected to fall by 6000, while the RAF will lose 5000 staff.
Gates, who served under presidents Barack Obama and George Bush, told BBC Radio 4's Today program that naval cuts were particularly damaging, noting that for the first time since World War I, Britain does not have an operational aircraft carrier.
He said: "With the fairly substantial reductions in defence spending in Great Britain, what we're finding is that it won't have full spectrum capabilities and the ability to be a full partner as they have been in the past."
This referred to the ability to fight on land, sea or in the air.
His concerns echoed those of senior military staff in the UK.
Last month General Sir Nicholas Houghton, the Chief of the Defence Staff, warned that manpower was increasingly seen as an "overhead", and that Britain was in danger of being left with hollowed out armed forces, with "exquisite" equipment but without the soldiers, sailors and airmen needed to man it.
He told the Royal United Services Institute military think-tank that the Royal Navy was "perilously close" to its "critical mass" in terms of manpower.
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