The Commons Defence Committee said the ending of military operations in Afghanistan combined with a reduced appetite for overseas interventions could lead the public questioning the purpose of the forces.
It expressed concern that defence would be increasingly seen as a matter of "discretionary spending" and warned that any further cuts would result in a "disproportionate decline" in the forces' fighting power.
It was essential, it said, that future decisions relating to Britain's ability to deploy forces around the world were based on proper strategic thinking and not simply the political "horse-trading" which accompanies the Government's comprehensive spending review process. The committee called on ministers to develop a proactive communications strategy to bridge the "disconnect" with the public and explain why military force was still needed.
It said: "Explaining the case for defence to the public only becomes harder in the light of public scepticism about both the objectives of recent operations and how success in them might be defined. There is a lack of understanding amongst the public of what HM Armed Forces should be for, and this represents one of the greatest strategic threats facing the armed forces.
"Public sympathy and support is to be welcomed, but it must not obscure or undermine a hard-headed understanding of what they are for."