Half of all alcoholics and drug addicts are suffering from mental health problems, including depression, and are self-medicating to mask the symptoms.
Turning Point, a social care charity, is warning that people are failing to get proper treatment because they are not being correctly diagnosed. It believes that mental health problems are being exacerbated by drug and alcohol misuse.
The problem of "dual diagnosis" is so acute that doctors' surgeries and hospitals are to be given official guidance to help them to identify people abusing drugs or alcohol who are also mentally ill.
Ministers have commissioned Turning Point to produce a good practice guide for doctors and nurses to help them to identify people with the symptoms of mental illness.
The move comes amid growing warnings from health-care experts that more needs to be done to provide support for the record numbers of Britons now suffering from depression and anxiety.
Figures released by the Government show that almost 13 million working days are lost every year through people taking time off because of stress, depression or other forms of mental illness.
Mental health problems are particularly common among homeless people and the prison population. Seventy-nine per cent of men on remand who use drugs also suffer from mental health disorders.
Lord Adebowale, the chief executive of Turning Point, said he was concerned that the medical services were missing symptoms of mental illness among drug and alcohol users.
"I would argue that the majority of drug addicts and alcoholics have a mental health problem," he said.
Experts believe that many who are abusing alcohol and drugs are unaware they are suffering from a form of mental illness.
According to the Royal College of Psychiatrists, approximately half of those using drug and alcohol services have some form of mental health problem, most commonly depression or personality disorder.
It is estimated that about one-third of psychiatric patients with severe mental illness have a substance misuse problem.
Tim Loughton, the shadow Health Minister, said: "The correlation between mental illness and alcohol abuse is very defined. Those who are depressed may self-medicate with alcohol in order to relieve their symptoms. Indeed, alcohol dependency is a type of mental illness. But mental health treatment is currently the Cinderella of the NHS."
Another concern is the risk of suicide among people with mental health problems. Marjorie Wallace, chief executive of the charity Sane, asked: "Why are we sending people who have attempted suicide back to their lonely flats with an outpatient appointment four weeks ahead?"