Analysis of 20 years of Danish medical and hospital data found that while suicide, accidents including car crashes, and homicide were major contributing factors in the 10-year life expectancy gap, those causes of death had seen some decline.

But ANU School of Demography associate professor Vladimir Canudas-Romo said he was surprised to see the gap in years lost between people with and without mental disorders had increased for deaths due to cancer and cardiovascular disease.

There was also a marked increase in the number of alcohol-related deaths, he said.

"Overall the mortality gap has stayed the same, but the causes of death have changed," Dr Canudas-Romo said.

"Cancer and health issues such as diabetes and heart disease are now playing a bigger role."

Overall, the analysis found men with a diagnosed mental illness could expect to live 10.2 years less than people without, while the lives of women with mental illness were 7.3 years shorter.

Lead researcher Annette Erlangsen from the Danish Research Institute for Suicide Prevention said the study showed the mortality gap had stayed the same despite concerted efforts to address the issue.

"It is worrisome that the mortality gap between people with mental disorders and the general population has not decreased over the past decades - despite our efforts to address suicide prevention and other relevant factors," she said.

Dr Canudas-Romo is exploring replicating the study in Australia.

The article - Cause-specific life-years lost in people with mental disorders: a nationwide, register-based cohort study - was published in The Lancet Psychiatry.