0646 GMT December 17, 2018
Most sufferers were veterans who saw active combat; 17 percent reported symptoms of probable PTSD, BBC wrote.
Experts said the delayed onset of the illness, and the loss of support when leaving the army, were probable causes.
And more veterans are seeking treatment, as awareness of PTSD has increased.
The study of nearly 9,000 of the military, by King's College London, is published in the British Journal of Psychiatry. It showed that PTSD in the military increased from four percent in 2004-5 to six percent in 2014-16.
Among veterans deployed in a combat role to Iraq or Afghanistan, 17 percent reported symptoms suggesting probable PTSD, compared to six percent deployed in support roles such as doctors and aircrew.
Lead author Dr. Sharon Stevelink, from the Institute of Psychology, Psychiatry & Neuroscience (IoPPN) at King's College, said: "For the first time we have identified that the risk of PTSD for veterans deployed in conflicts was substantially higher than the risk for those still serving.
"While the increase among veterans is a concern, not every veteran has been deployed and in general only about one in three would have been in a combat role."
The findings are from the third phase of a major study which has been running since 2003.
The latest phase of the study surveyed participants between 2014 and 2016. 62 percent of them had deployed to Iraq or Afghanistan, and the average age was 40.
The study also found that common mental health disorders — like anxiety and depression — had remained largely static at 22 percent.
Explaining the high figures among veterans, Prof. Nicola Fear from the IoPPN, pointed out that people with mental health issues are more likely to leave the army. The act of leaving, with the corresponding loss of social support, can also trigger conditions.
"We know that individuals who experience mental health issues are more likely to leave the armed forces.
"We also know that the transition of leaving the armed forces can introduce stresses. You may need to find somewhere to live, you may need to find a job."
There is also far greater awareness of PTSD and mental health issues generally, popularized by dramas like BBC TV's Bodyguard, and many veterans feel more comfortable talking about their condition.
PTSD affects between four percent to five percent of the UK population, with the highest rate of 12 percent among women between 16-24 years old. Victims of sexual assault have the highest rates of PTSD.
Prof. Simon Wessely, Regius Professor of Psychiatry at King's College London, pointed out that treatments have improved and there's less stigma attached.
"It used to take people maybe 10-13 years to come forward for treatment, now that's down to around 2-3 years," he said.
"There's been a huge investment in treatment, with many more services available.
"When people leave they can lose the social network that the Army provides. Support from family and friends can be crucial."
Professor Wendy Burn, President of the Royal College of Psychiatrists, said: "It is vital that we meet the growing demand for military and veteran mental health services so that those who serve their country can easily access care."