The Government needs to give the armed forces a "clear and honest" picture of the UK's military ambitions, three senior retired commanders have said.
In a report for the United Kingdom National Defence Association, Air Chief Marshal Sir Michael Graydon, General Sir Michael Rose and Vice-Admiral Sir Jeremy Blackham, said current spending on the armed forces was "incoherent".
The Funding Defence report, which has been written by the three retired military leaders, said spending on defence could be used to provide an "excellent economic stimulus".
But, the report said, the loyalty and service of Britain's military "cannot be assumed forever".
It added: "It has to be earned by the Government through a long-term commitment to the careers and well-being of the personnel of the armed forces.
"The challenge of how much to spend on defence lies in a clear and honest view of our ambitions and obligations for today and the future."
Their comments come as Defence Secretary Philip Hammond warned the armed forces cannot take any more large-scale spending cuts.
Speaking last week, he said: "I have never said, and I would never say, that there are not further efficiencies that we can take and we are working right now to identify where we might find still further efficiencies across the defence budget.
"But really, if we go beyond what can be delivered from efficiency, there will have to be a reassessment of the output that is delivered."