NHS managers in England have been accused of "shocking discrimination" in commissioning mental health services.
The Mental Health Policy Group from the London School of Economics said three-quarters of people with depression or anxiety got no treatment.
The committee of senior academics and medical professionals described this as a "real scandal".
Care Services Minister Paul Burstow said mental health should be treated as seriously as physical health issues.
The committee is headed by economist Professor Lord Richard Layard and includes some of the country's most eminent mental health experts.
It said the NHS in England was guilty of injustice in its treatment of people with mental illness.
The group's report found that among those aged under 65, nearly half of all ill-health was mental illness.
It said six million people had depression or anxiety conditions and yet three-quarters got no treatment.
This was often because NHS managers failed to commission properly the mental health services recommended in official guidance, the experts added.
They said £400m earmarked by the government for psychological therapy was not always used for its intended purpose because there was no obligation on managers to do so.
The committee concluded that mental health services should be expanded, but if anything it was being cut.
Lord Layard said: "If local NHS commissioners want to improve their budgets, they should all be expanding their provision of psychological therapy.
"It will save them so much on their physical healthcare budgets that the net cost will be little or nothing.
"Mental health is so central to the health of individuals and of society that it needs its own cabinet minister."
Mr Burstow said investment in mental health services was "already delivering remarkable results".
"Mental ill-health costs £105bn per year and I have always been clear that it should be treated as seriously as physical health problems," he said.
"We will shortly publish our plans to make sure the NHS, councils, voluntary organisations and others can play their part in improving the nation's mental health.
"The coalition government is investing £400m to make sure talking therapies are available to people of all ages who need them."