The finding comes despite 80% agreeing that serving personnel make a valuable contribution to society.
"Whilst it’s fantastic that people think our Armed Forces make a valuable contribution to society, the lack of understanding of what they actually do is concerning," Alexander Owen, Head of Armed Forces Engagement at The Legion said.
"If this figure continues to decline, I worry that support for those who serve will diminish too."
The study carried out by YouGov found that:
A similar poll carried out by Lord Ashcroft in 2012 found that 62% of UK adults had little or very little understanding of what the Armed Forces do on a day-to-day basis, a figure which has increased to 69% of UK adults in the YouGov research today.
In response to these findings we are raising awareness of the contribution that the Armed Forces make to everyday life; Serving personnel are mobilised during natural disasters, terrorist attacks, humanitarian and environmental crises and as a peacekeeping force around the world.
"Not only do they protect our values, freedoms and way of life but they work tirelessly supporting the NHS, respond to natural disasters and civilian emergencies at home and abroad, and are currently helping protect endangered species for future generations," Alexander Owen added.
"They support us every day. The least we can do is support them in turn."