An analysis by Oxford University psychiatrists has shown that serious mental illnesses reduce life expectancy by 10-20 years - a loss of years that's equivalent to or worse than that for heavy smoking.

Yet mental health has not seen the same public health priority, despite these stark figures and the similar prevalence of mental health problems, the Oxford scientists said.

1 in 4 people in the UK will experience some kind of mental health problem in the course of a year, it is estimated. Around 21 percent of British men and 19 percent of women smoke cigarettes.

The researchers said that the figures should galvanise governments and health and social services to put a much higher priority on how mental health services can prevent early deaths.

The average reduction in life expectancy in people with bipolar disorder is between 9 and 20 years, it's 10-20 years for schizophrenia, between 9 and 24 years for drug and alcohol abuse, and around 7-11 years for recurrent depression.

The loss of years among heavy smokers is 8-10 years.

All diagnoses studied showed an increase in mortality risk, though the size of the risk varied greatly. Many had risks equivalent to or higher than heavy smoking (see table in notes for editors).

"We found that many mental health diagnoses are associated with a drop in life expectancy as great as that associated with smoking 20 or more cigarettes a day," Dr Seena Fazel of the Department of Psychiatry at Oxford University said.

"There are likely to be many reasons for this. High-risk behaviours are common in psychiatric patients, especially drug and alcohol abuse, and they are more likely to die by suicide. The stigma surrounding mental health may mean people aren't treated as well for physical health problems when they do see a doctor," Dr Fazel said.

The findings are published in the open access journal World Psychiatry.