London - Britain's military cuts mean it will no longer be a full partner alongside United States forces, former US defence secretary Robert Gates said on Thursday.
Gates, who served under US presidents Barack Obama and George W Bush, said Britain no longer had the complete spectrum of capabilities, meaning its relationship with the US military was shifting.
Prime Minister David Cameron said Gates had "got it wrong" and said Britain remained a "first-class player" globally.
Gates, speaking to BBC radio, highlighted the Royal Navy's lack of an aircraft carrier able to launch strike jets.
"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," Gates said.
"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.
"I also lament that reality."
The comments suggest Britain's military downsizing could have a negative impact on the long-standing "special relationship" between Washington and London.
This century, Britain has been the United States' closest partner in the wars in Iraq and Afghanistan.
Cameron's coalition government announced heavy defence cuts in 2010 as part of its bid to rein in Britain's massive deficit.
The defence budget is being slashed by eight percent over four years.
But Cameron insisted Britain's military was still in the front rank.
"I don't agree with him. I think he has got it wrong," he said of Gates.
"We have the fourth-largest defence budget anywhere 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 minister that is the way it will stay."