David Cameron yesterday dismissed a warning from ex-US defence secretary Robert Gates that armed forces cuts would diminish the UK's military standing.
The prime minister said Britain has the world's fourth largest defence budget and was a "first class-player in terms of defence".
The UK plans to cut 30,000 armed forces personnel by 2020, leaving 147,000.
Gates said the erosion in Britain's capabilities had reduced its ability to be a "full partner" to the US.
He told BBC Radio 4's Today programme the "fairly substantial reductions" in UK defence spending meant it "won't have full spectrum capabilities" - meaning the ability to fight on air, land and sea - that it had previously.
Cameron, however, said Gates was "wrong", adding that Britain was investing in its "future capabilities".
"We've got a massive investment programme of £160bn in our defence industries, in our equipment," he said.
"We are a first-class player in terms of defence and as long as I am prime minster that is the way it will stay."
The Ministry of Defence said the UK - like the US - had had to take "tough decisions" on defence spending but had "the best trained and best equipped armed forces outside the US".
Gates, who served under presidents Obama and Bush, singled out cuts to the Royal Navy as particularly damaging and he noted that - for the first time since World War One - Britain did not have an operational aircraft carrier.
Gates warned against nuclear disarmament by the UK, but acknowledged that there was scope for changes to the way the deterrent was deployed.
Under the government's proposals, by 2020 the Army will lose 20,000 soldiers, the Royal Navy 6,000 personnel and the RAF 5,000 - although there are also plans to increase the number of reservists in each service.
Gates's comments echo the concerns of senior military figures in the UK.