Members of the armed forces who bore the brunt of the wars in Iraq and Afghanistan are suffering a mounting toll of post-traumatic stress, alcoholism and family breakdown, an extensive examination of mental health in the military has found.

The study sampled 5,500 service personnel, focusing on the 20% deployed for more than 13 months over a three-year period between 2001 and 2006 - the maximum term set by government guidelines.

Researchers at King's College London found those in this category were more likely to have symptoms of post-traumatic stress disorder and problems at home during and after deployment. This was particularly apparent in those with direct combat exposure.

The study, funded by the Ministry of Defence and published in the British Medical Journal, found that one in four had "severe" alcohol problems, compared with 10% of those deployed for less than five months.

Post-traumatic stress disorder was found in 5.2% of those deployed above the 13-month limit, compared with 3% of those who spent less than five months in conflict zones.

The study covered deployments since 2000 and included tours of duty in Kosovo and Sierra Leone, as well as Afghanistan and Iraq. The researchers found that those deployed for longer and those who were uncertain about when they would return home were more likely to suffer mental distress.

The study concluded: "A clear and explicit policy on the duration of each deployment of armed forces personnel may reduce the risk of post-traumatic stress disorder. An association was found between deployment for more than a year in the past three years and mental health that might be explained by exposure to combat."

The report comes after warnings that the navy, army and Royal Air Force were overstretched in Iraq and Afghanistan. The study found that 86% of those who had been deployed in the previous three years had spent time in Iraq.

A spokesman for the Ministry of Defence said the latest figures showed that less than 1% of Royal Navy, 12% of army and 6% of RAF personnel were exceeding the individual service's harmony guidelines of 13 months in combat over a three-year period.

The number of people exceeding the harmony guidelines was coming down, he said.

The under-secretary of state for defence, Derek Twigg, said: "We recognise mental illness as a serious and disabling condition and we want to ensure that our people have the best possible mental health support. This is why the MoD commissioned research to monitor the health of those deployed on operations.

"We have taken great steps to raise awareness and to reduce the stigma associated with mental illness. These include pre- and post-deployment briefing and the availability of support, assessment and, if required, treatment - both during and after deployments. We are studying the research to see how we can reduce even further the number of troops who suffer from mental illness.

"We have always been open that we have a high operational tempo, but as the chief of defence staff has said in the past we are stretched but not overstretched."

Mr Twigg said the vast majority of British troops did not have their tours extended and were on operations for no longer than six to seven months.

"As such, it indicates that our current policies on the duration of tours are right. We are taking steps to remind the small number of troops who see their tours extended about the support that is available to them. We have, for example, mental health nurses in Iraq and Afghanistan. I would encourage service personnel who think that they might be suffering from mental illness related to their service to seek professional help."