Thousands of people with mental health problems "die needlessly" in England each year due to a failure to take the problem seriously, a charity says.
Rethink Mental Illness said smoking, drinking and obesity were leading to 33,000 avoidable deaths each year.
The Royal College of Psychiatrists said it was "one of the biggest health scandals of our time".
Ministers said they expected mental health to be treated with as much importance as physical health.
One in six people in the UK are considered mentally ill, ranging from those with depression, which is relatively common, to those with rarer disorders such as schizophrenia.
In England, more than 100,000 deaths each year are officially classed as "avoidable", often due to lifestyle decisions such as a bad diet.
This includes thousands of deaths from lung cancer that could have been prevented if people did not smoke. Heart problems due to a lack of exercise, a diet packed with fatty food and smoking are also a major contributor to the figures. Suicide is not counted in the statistics.
Rethink said people with mental illnesses represented a large proportion of the avoidable deaths figures, but the issue was being ignored.
It said patients were less likely to be given support to help them stop smoking, despite 40% of cigarettes being smoked by people with mental illness.
It also said some anti-psychotic medication led to patients putting on a stone in weight, but this was often not monitored.
Victoria Bleazard, associate director of campaigns at Rethink, criticised Health Secretary Jeremy Hunt for "barely touching on mental health" when announcing plans to deal with avoidable deaths earlier this year.
She said there were "systemic problems in the NHS" including doctors focusing on a patient's mental health problems, such as bipolar disorder, and not dealing with physical health problems and an attitude that "smoking is the last treat they've got" so should not be tackled.
In the report's foreword, Prof Sue Bailey, president of the Royal College of Psychiatrists, described the figures as "chilling".
She said: "The fact that people with serious mental illness [such as schizophrenia] die an average of 20 years earlier than the rest of the population is one of the biggest health scandals of our time, but it is being ignored.
"If this statistic applied to any other group of people, such as residents of a particular town, there would be public outcry. This simply isn't happening for people with mental illness.
"Failure to address this issue amounts to a form of lethal discrimination which is costing lives."
Paul Jenkins, the chief executive of Rethink Mental Illness, said: "If Jeremy Hunt is serious about wanting to reduce the number of people dying needlessly every year, he can't afford to ignore people with mental illness.
"We know this group is at a much higher risk of early death. We also know that there are simple solutions like targeted support to give up smoking and regular physical health checks, which could save thousands of lives."
Care and Support Minister Norman Lamb said: "This government has made improving the overall health of people with mental health problems a bigger priority than ever before, and we're determined that mental health is treated with as much importance as physical health in the NHS.
"We have made it clear that we expect the NHS to reduce the number of premature deaths in people with mental health problems.
"Later this year, we will also be publishing a five-year action plan on how to reduce avoidable deaths, including for people with mental health problems."