Black African or Caribbean people are three times more likely to be hospitalised for mental health problems than the average, a new survey finds.
A national census of inpatient care finds this group is also up to 44 per cent more likely to be detained under the Mental Health Act - when someone is hospitalised for their own safety.
The joint report from the Healthcare Commission, the Mental Health Care Commission and the National Institute for Mental Health in England offers no explanation for this disparity.
However, the census does not take into account socio-economic and clinical differences between ethnic groups, and the report admits these might affect the results.
"There is an association between the likelihood of mental illness and social and economic issues such as poverty, poor education achievement, unemployment and inner city living," it says.
"Deprivation, racism and discrimination, as well as family, and social support networks can also affect the course of mental illness, and the outcomes of care."
The report insists that an explanation must be found as a matter of urgency, with the authors warning: "It would be wrong and intolerable if someone were categorised as mentally ill and hospitalised solely on the basis of colour or ethnic origin.
"It would be equally wrong and intolerable if someone who is mentally ill and would benefit from care in hospital did not have that benefit because those charged with such decisions were anxious that they may be accused of racial prejudice."
The survey, which took place in March, covered almost all mental health inpatients in the NHS and is the first time such a comprehensive survey has been done.
Mental health charity MIND said the results confirmed its concerns about the treatment of black patients, and highlighted in particular the finding that black men were 29 per cent more likely to be subject to control and restraint.
"For some time, black service users have been telling us of the difference in their treatment," said policy director Sophie Corlett.
"The census demands some urgent answers to what seems to be routine subjection of black people to more extreme measures than white people."
Health minister Rosie Winterton said the findings would act as a benchmark for future policies and help eliminate inequalities in mental health services.
"Racism or discrimination in any form have no place in modern health or social care - they are an affront to the core values of the NHS," she said.
The minister added: "I know that many services around the country are improving the care they provide for ethnic minority communities and I am confident that we have started to build the sort of service that can make the inequalities that the census confirms a thing of the past."