The Tories have been dealt a pre-general election blow after a top ranking US army head warned that the UK is not spending enough on its armed forces.
The British government is currently committed to spending 2% of its GDP on defence.
But, as part of George Osborne's spending cuts, the Army will shrink from 102,000 in 2010 to 82,000 by 2018.
Raymond Odierno, US chief of staff general, warned that UK army units may have to work within American divisions in the future.
"I would be lying to you if I did not say that I am very concerned about the GDP investment in the UK," the 60-year-old veteran said.
"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.
"We have to adjust our programme to make sure we are all able to see that we can still work together."
Odierno also told the Daily Telegraph that the UK was an important ally to the US and "has very close values and the same goals as we do".
The comments come as Tory backbenchers push David Cameron not to reduce defence spending any further.
Former defence secretary Liam Fox warned that there could be problems within the Conservative Party if Cameron does not give a desirable defence spending commitment in the party's general election manifesto.
"We have to do what we need to make that happen and I think that we have a commitment to Nato as part of our international treaty obligations to spend that 2%," he said on BBC's Sunday Politics.
"I think to say that we were willing to guarantee a proportion of GDP for international aid but not willing to implement our commitments in terms of defence, I think a lot of Conservatives would find that very difficult to swallow – especially at a time when you can see that the international security environment is deteriorating."
A Ministry of Defence spokesman said: "Decisions on spending after the financial year 2015/16 will be determined in the next spending review.
"Over the next decade we are committed to spending £163bn ($250bn, €224bn) on equipment and equipment support to keep Britain safe.
"That includes new strike fighters; more surveillance aircraft; hunter killer submarines; two aircraft carriers; and the most advanced armoured vehicles."