Rates of probable PTSD among current and former serving military personnel reached six percent in 2014/16, up from four percent in 2004/2006, the King's College London study said.

The results of a major cohort study, published in the British Journal of Psychiatry, showed frontline soldiers were most affected, but so too were those who have since left the military.

Some 17 percent of ex-military personnel who had combat roles in Iraq or Afghanistan reported symptoms suggesting probable PTSD, compared to six percent of those who were deployed in support roles, such as logistics.

The overall rate of probable PTSD for veterans of the Iraq and Afghan wars was nine percent, compared to five percent of those who did not deploy to those conflicts.

Meanwhile the rate of probable PTSD among currently serving personnel was five percent, close to the rate among the general population.

"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," said Dr Sharon Stevelink from King's Institute of Psychiatry, Psychology and Neuroscience (IoPPN).

Professor Nicola Fear, also from the IoPPN, said: "One possible reason PTSD is more common among veterans may be that personnel who are mentally unwell are more likely to leave the armed forces."

The study, funded by the defence ministry, was carried out by the King's Centre for Military Health Research.

It began in 2003 and this phase included 8,093 participants, 62 percent of whom had been in Iraq, where British troops were deployed 2003-2011 or Afghanistan (2001-2014).