More than 30,000 people with men tal health problems are dying needlessly every year from causes other than suicide, according to new figures.
The Government estimates around 100,000 people under 75 die every year from illnesses that could have been prevented through better medical treatment or interventions like stop smoking campaigns.
A new report from the charity Rethink Mental Illness has found around a third of these deaths are among people with mental health problems.
They are more likely to die from preventable illness - such as cancer or heart disease - than the rest of the population.
The study, Lethal Discrimination, calls on the Government to do more to target this group, who have higher rates of illness through side-effects of medication, smoking, a lack of basic health checks on the NHS and professionals not taking their concerns seriously.
Figures show that people with mental illness are three times more likely to develop diabetes and twice as likely to die from heart disease.
More than 40% of all cigarettes are also smoked by people with mental illness, but research suggests they are less likely to receive support to quit.
Fewer than 30% of people with severe mental illness receive a basic annual physical health check.
Earlier this year, Health Secretary Jeremy Hunt issued a call to action to reduce premature death in England, mainly from the five biggest killers: cancer, stroke, heart, liver and respiratory disease.
Paul Jenkins, 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.
"While there are some pockets of good practice in the system, most people with mental illness are being badly let down when it comes to their physical health.
"This means tens of thousands of people are dying needlessly every year and many more are left struggling with long-term conditions such as diabetes.
"By not acting, the Government is allowing some of the most vulnerable people in our society to be treated as second class citizens.
"We know what the solutions are and they are not complex or expensive.
"All we need now is the political will to make change happen."
Key recommendations in the report include more specialist support to quit smoking, closer monitoring of patients for side-effects of medication and for clear guidelines on who is responsible for the physical health of people with mental health problems.
Professor Sue Bailey, president of the Royal College of Psychiatrists, who wrote the foreword for report, said: "The fact that people with serious mental illness 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.
"Questions would be asked about why these people are being so badly let down by health services and politicians would call for targeted support.
"But 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."
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."