The head of the US army has said he is "very concerned" about the UK's ability to meet its Nato defence spending commitments.
Speaking to the Telegraph, chief of staff General Raymond Odierno said the US was already planning for UK forces to operate "inside" American units rather than on their own terms.
"I would be lying to you if I did not say that I am very concerned about the GDP investment in the UK," he told the paper.
"As we look at threats around the world, these are global issues and we need to have multinational solutions”
"In the past we would have a British army division working alongside an American division.
"Now it might be a British brigade inside an American division, or even a British battalion inside an American brigade."
Nato members last year pledged to reverse the trend of declining defence spending, with Britain - which currently meets the target of spending 2% of GDP on defence - declaring it would "aim to continue" to reach that amount.
The Government has committed to meeting the pledge until the end of the current Parliament, but the Royal United Services Institute think tank has predicted that defence spending will fall below this level by 2015/16.
Fears the UK will not meet the target have prompted criticism from a number of senior Conservative backbenchers. Former defence secretary Liam Fox yesterday warned that his party would find it "difficult to swallow" a failure to ring fence the budget while international development remained protected.
This weekend, a DUP figure told the Financial Times the unionist party would insist that meeting the 2% pledge would be a "top silver bullet" in any potential Coalition negotiations.