Former U.S. defense secretary Robert M. Gates warned Thursday that defense cuts that Britain has undertaken have reduced the country’s capacity to cooperate fully with the United States in conflicts around the world.

“With the fairly substantial reductions in defense 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,” Gates said, speaking to the BBC.

British Prime Minister David Cameron rejected the assertions, saying that Gates “got it wrong,” and stressed that Britain was a “first-class player” in terms of defense.

Britain is among the United States’ staunchest military allies and has one of the most powerful armed forces in the world, but in recent years it has made cuts to its defense budget as part of the government’s overall reduction in spending. After a major review in 2010, plans were made to reduce the armed forces by 30,000 personnel by 2020. Britain’s only aircraft carrier also was scrapped.

Gates lamented defense cuts on both sides of the Atlantic, but he singled out that last move as an example of the shift in Britain’s military relationship with the United States.

“They can be a full partner and probably will because we have a long history of doing this,” Gates said. “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 I, Great Britain will not have an operational aircraft carrier.”

“Those kinds of things, I think, at the end of the day, matter,” he said.

Britain is building a new aircraft carrier that it plans to put into service in 2020.

The British defense community also has voiced concern about the cuts. Last month, Gen. Nicholas Houghton, chief of the defense staff, said that loss of manpower put Britain in danger of being left with “exquisite” equipment without the personnel to operate it.

But the views of Gates, who is promoting his new memoir, “Duty,” have garnered particular attention.

“When a former U.S. defense secretary says something like that, it really hits home,” said Richard Kemp, a former commander of British forces in Afghanistan. “A major criticism from outside is sometimes more noteworthy than criticism from the inside.”

In a statement, Britain’s Defense Ministry said: “Like the United States, the U.K. 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 U.S.”