Spending cuts have reduced the UK 's ability to be a "full partner" to the United States across the whole range of military operations, a former US defence secretary has said.
Robert Gates highlighted the decision which has left the Royal Navy without an operational aircraft carrier as an example of the erosion of "full spectrum capabilities" in the British armed forces.
Mr Gates, who served under presidents Barack Obama and George Bush, said he lamented cuts in defence spending on both sides of the Atlantic.
He told BBC Radio 4's 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: "T hey 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.
"Those kinds of things, I think, at the end of the day matter."
Setting out his support for continued co-operation he said: "I t's always better from the standpoint of the United States to have the flags of other nations part of those efforts to deter others. Our primary partner for many decades now has been the United Kingdom.
"I would a lot rather, in a port in the Middle East, to have a British-flagged ship and an American-flagged warship than just an American-flagged warship by itself."
Mr Gates warned against nuclear disarmament by the UK, but acknowledged that there was scope for changes to the way the deterrent was deployed.
The Liberal Democrats have campaigned within Government for an alternative to the like-for-like replacement of the Trident system, and have argued for a reduction in the number of submarines and an end to round-the-clock patrols.
Mr Gates said: "This is an area where Britain having an independent capability has been an asset and a strength for the United States. As we reduce the size of our nuclear arsenal we potentially get down to numbers that having allies who have their own capability could be very helpful."
But he added: "That does not rule out changes in the way that it's deployed and in the numbers in which it's deployed. The UK will have to make its own decisions on that score, I'm just saying that I think it's important for the UK not to deny itself this military capability altogether."
He questioned the way cuts in defence spending had been implemented in the West at a time of global uncertainty.
"If we are to reduce our military capabilities, as we are, I think it ought to be done much more gradually and with a wary eye to the global environment, not willy-nilly as a matter of just making the budget numbers work."
Former UK defence secretary Liam Fox, who was responsible for the 2010 review, said the cuts had not damaged the relationship with the US.
He told Today: "As Bob Gates was saying the United States is making substantial cuts of around 500 billion dollars (£305 billion) to the Pentagon budget over this decade.
"America is the world's biggest economy, the world's biggest defence budget. We are the world's eighth biggest economy, with the fourth biggest defence budget, so certainly we are pulling our weight on that.
"We are one of only four or five countries inside Nato to meet our 2% spend commitment, so I don't think we can be questioned on that.
"But it's self-evident to say that all capabilities depend on budget and both the United States and United Kingdom are facing budgets where we are paying a huge amount of debt interest on the debts we have."
Last month General Sir Nicholas Houghton, the Chief of the Defence Staff, warned 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.
He told the Royal United Services Institute military think-tank that the Royal Navy was "perilously close" to its "critical mass" in terms of manpower.
Dr Fox said: "What General Houghton said is if we went further than the current plans which were set out in the last defence review then that could result in hollowing out - I've got some sympathy with that view.
"But of course generals don't have to raise taxes, which governments do and we have to arrange our priorities accordingly."
Shadow defence secretary Vernon Coaker said confidence in Britain's defence commitment was being damaged and singled out current Defence Secretary Philip Hammond for blame.
He said : "It should worry David Cameron that Britain's strongest ally has concerns about his Government's mishandling of defence.
"Philip Hammond's botched procurement reforms, the mess he's made of armed forces reform and the latest fiasco over the MoD IT system are damaging confidence in Britain's commitment to defence and our ability to continue to play a significant role in the world.
"It's clear that there needs to be a rebalancing after withdrawal from Afghanistan and the closure of bases in Germany.
"And no one is disputing the financial constraints within which the UK military must operate. But the Government must ensure that Britain's defence capability is maintained.
"Labour will ensure that the next Strategic Defence and Security Review in 2015 will be strategy-led and financially responsible."
A Ministry of Defence 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.
"Over the next decade, we plan to spend £160 billion on equipment, to ensure our armed forces retain their formidable range of cutting-edge capabilities and ability to project power across the globe, including by maintaining our major naval presence in the Gulf region.
"Construction of the first new aircraft carrier, the largest ship the Royal Navy has ever operated, is almost complete and the Navy will also operate state-of-the-art type-45 destroyers, new type-26 frigates, and seven new Astute class submarines."