Black people are three times more likely than the rest of the population to be admitted to mental hospitals in England and Wales. They are twice as likely to be sent there by the police or the courts, and 50% more likely to be placed in seclusion, according to the first ethnic census of inpatients, published today by the health inspectorate.

The study was triggered by allegations of institutional racism in mental health services made last year by Sir John Blofeld, a retired high court judge who chaired the independent inquiry into the death of David "Rocky" Bennett, who died in Norwich in 1998 after being held face down on the floor for 28 minutes by at least four mental health nurses.

The judge blamed the Department of Health for poor standards of treatment that led to black people being detained in mental hospitals because of a lack of timely care in the community. He described this as a "festering abscess" and "blot on the good name of the NHS".

Health ministers rejected his allegation of institutional racism, but ordered ethnic monitoring of all 34,000 inpatients in trusts in England and Wales. The census was conducted on March 31 by the Healthcare Commission and mental health authorities.

Sir Ian Kennedy, the commission chairman, refused to be drawn last night on whether the findings proved there was discrimination in the system. "This census demands an explanation. It does not provide one. The job of discovering the reasons behind the data must be undertaken with urgency. Until these reasons become clearer, it is unwise to draw premature conclusions," he said in a joint statement with Kamlesh Patel, chairman of the Mental Health Act Commission.

"It would be wrong and intolerable if someone was 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 they might be accused of racial prejudice," they added.

The census found 9% of mental health inpatients were black or of mixed white-black ethnicity. These groups made up 3% of the national population in 2001. Black inpatients were 44% more likely to have been sectioned under the Mental Health Act and 50% more likely to be put in seclusion. Black Caribbean men were 29% more likely to have been subjected to control and restraint.

Angela Greatley, of the Sainsbury Centre for Mental Health, said: "This is a glaring inequality in health care. African and Caribbean people are being admitted to hospital as a first resort before other courses of action are considered."

Rocky Bennett's sister, Joanna Bennett, said: "This census does not tell the black community anything. It may satisfy the sceptics about inequality in mental health services, but it doesn't help service users. Resources should be used to improve black people's experiences of services instead of providing more evidence of the problem and funding high-flying posts for so-called black experts."

Paul Farmer, director of the mental health charity Rethink, said: "The daily experience of using mental health services for far too many black people is of exclusion, compulsion and racism ... the mental health bill simply can't be published in its present form until these issues are fully addressed."

Health minister Rosie Winterton said: "Racism or discrimination in any form have no place in modern health or social care ... The results of the census will act as a benchmark for measuring our progress in ending those inequalities."