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% of British men and 19% of women smoke cigarettes.
The researchers say 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 Oxford University researchers report their findings in the open access journal World Psychiatry. The study was funded by the Wellcome Trust.
The researchers searched for the best systematic reviews of clinical studies which reported mortality risk for a whole range of diagnoses - mental health problems, substance and alcohol abuse, dementia, autistic spectrum disorders, learning disability and childhood behavioural disorders. 20 review papers were identified, including over 1.7 million individuals and over 250,000 deaths.
They repeated searches for studies and reviews reporting life expectancy and risk of dying by suicide, and compared the results to the best data for heavy smoking.
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).
Dr Seena Fazel of the Department of Psychiatry at Oxford University said: '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.
'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.'
One problem is the tendency to separate mental and physical illness, explains Dr Fazel. 'Many causes of mental health problems also have physical consequences and mental illness worsen the prognosis of a range of physical illnesses, especially heart disease, diabetes and cancer. Unfortunately, people with serious mental illnesses may not access healthcare effectively,' says Dr Fazel.
Dr Fazel is certain: 'All of this can be changed. There are effective drug and psychological treatments for mental health problems. We can improve mental health and social care provision. That means making sure people have straightforward access to health care, and appropriate jobs and meaningful daytime activities. It'll be challenging, but it can be done.'
He notes: 'Beyond that, psychiatrists have a particular responsibility as doctors to ensure that the physical health of their patients is not neglected. De-medicalization of psychiatric services mitigates against that.'
He adds: 'What we do need is for researchers, care providers and governments to make mental health a much higher priority for research and innovation. Smoking is recognised as a huge public health problem. There are effective ways to target smoking, and with political will and funding, rates of smoking-related deaths have started to decline. We now need a similar effort in mental health.'
Dr John Williams, Head of Neuroscience and Mental Health at the Wellcome Trust, said: 'People with mental health problems are among the most vulnerable in society. This work emphasises how crucial it is that they have access to appropriate healthcare and advice, which is not always the case. We now have strong evidence that mental illness is just as threatening to life expectancy as other public health threats such as smoking.'