Britain's military cuts mean it will no longer be a full partner alongside United States forces, former US defence secretary Robert Gates said 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.

Between 2010 and 2020, Britain is reducing the size of its regular military from 178,000 to 147,000, while boosting reservist numbers.

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."

Britain has not been able to launch jets at sea since the flagship Ark Royal was withdrawn in 2010 as part of the cuts.

It will not be able to do so before the new aircraft carrier Queen Elizabeth -- which will be Britain's biggest-ever ship on the seas -- enters service in 2020 with F-35 fighter jets.

Gates said Britain had been the United States' "primary partner for many decades".

However, with the lack of carrier strike capability, he added: "Those kinds of things, I think, at the end of the day matter.

"I would a lot rather, in a port in the Middle East, have a British-flagged ship and an American-flagged warship than just an American-flagged warship by itself."

He also urged Britain to maintain a nuclear deterrent.

The coalition has deferred a decision on replacing Britain's nuclear weapons programme -- the Trident missile-based system on board its submarines -- until after the 2015 general election.

"I think it's important for the UK not to deny itself this military capability altogether," Gates said.

A British Ministry of Defence spokesman said: "We still have the fourth-largest defence budget in the world and the best-trained and best-equipped armed forces outside the United States."