Britain is a "first-class player" in defence terms, David Cameron said yesterday as he dismissed claims by a former US defence secretary that spending cuts have left the UK diminished on the world stage as "wrong".
Robert Gates said the erosion in Britain's capabilities had reduced its ability to be a "full partner" to the United States across the whole range of military operations.
But the Prime Minister insisted that the UK's defence budget remained the fourth largest globally and more investment was being made to enhance its capabilities.
During a visit to the Crossrail project in central London, Mr Cameron said: "I don't agree with him. I think he has got it wrong. We have the fourth largest defence budget in the world. We are investing in future capabilities.
"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."
It comes after General Sir Nicholas Houghton, the chief of the defence staff, warned last month 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.
Mr Gates, who served under presidents Barack Obama and George W Bush, said he lamented cuts in defence spending.
He told the BBC Radio 4 Today programme: "What we have always been able to count on, on this side of the Atlantic, were British forces that had full-spectrum capabilities very much along the lines of our own forces, that they could perform a variety of different missions.
"With the fairly substantial reductions in defence spending in Great Britain, what we are finding is they won't have full-spectrum capabilities and the ability to be a full partner as they have been in the past. Because I lament our own defence cuts, I would say I also lament that reality in terms of Great Britain."
The decisions in 2010's Strategic Defence and Security Review left the UK without an operational aircraft carrier until the new Queen Elizabeth enters service in 2020.
Mr Gates said this was the kind of decision that had reduced the UK's ability to support the US: "They can be a full partner, and probably will because we have a long history of doing this. What I'm saying is the capabilities to do the full spectrum of military operations will be limited with these plans – for the first time since World War One, Great Britain will not have an operational aircraft carrier."
Shadow defence secretary Vernon Coaker said: "It should worry David Cameron that Britain's strongest ally has concerns about his Government's mishandling of defence."
An MoD spokesman said: "Like the United States, the UK has had to take tough decisions on defence spending, but we still have the fourth largest defence budget in the world and the best-trained and best-equipped armed forces outside the US."