There were around 600 deaths in custody in England and Wales last year, a third of them suicides, according to a report published today.
The study, by the Forum For Preventing Deaths In Custody, found there were 500 to 600 deaths in custody each year, some of which were preventable.
The figures - covering deaths in police cells, prisons, approved premises and secure hospitals - included deaths from natural causes, suicides and other events such as homicides.
Around 400 of the deaths each year were due to natural causes and 200 were self-inflicted, a spokeswoman for the Ministry of Justice said.
This makes the suicide rate in custody around 33 times greater than that in the general population. Suicides account for just 1% of deaths in people aged over 15 in the UK.
In 2004-05 - the most recent year with a full breakdown of the cause of deaths in custody - 127 of the 590 deaths in custody were suicides.
There were 523 deaths in custody in 2006-07 but this figure did not include deaths in police custody. In 2005-06 there were 586 deaths.
The number of suicides in prisons alone fell from 78 in 2005 to 67 last year, but the figure is on the rise again this year. There have been 68 jail suicides so far in 2007 compared with 46 at the same point last year. Prison reform campaigners have blamed the rise on record levels of overcrowding.
John Wadham, the chairman of the Forum For Preventing Deaths In Custody, said: "The number of deaths in custody is the mark of a civilised society. I think this is too high and we need to reduce it."
He said individual institutions had learned lessons from mistakes that had contributed to deaths on their premises, but these lessons were not being shared across the board.
One example was that not all institutions had taken adequate steps to remove ligature points from cells in order to help stop inmates hanging themselves.
Mr Wadham warned that the report had found suicidal people could be "very inventive about where they can attach ligatures".
The forum was established after the parliamentary joint committee on human rights called for the Home Office and the Department for Health to set up a body to monitor deaths in any form of state custody, including mental hospitals. Its aim is to spread best practice and information on preventing custody deaths.
The report also raised concerns about the number of mentally ill people in custody, and suggested they would be better looked after in psychiatric care.
Juliet Lyon, the director of the Prison Reform Trust, agreed that mentally ill people should receive healthcare instead of being imprisoned.
But she blamed the rise in the number of suicides in all custodial setting last year on overcrowding.
Ms Lyons, who described the rise in self-inflicted deaths as "very disappointing", said overcrowding was preventing prison officers from closely monitoring vulnerable inmates.
"The prison service have been working rather well to try and reduce risk and respond better to vulnerable people," she said.
"But the level of overcrowding now is such that a lot of those efforts have been swept away," she told the BBC Radio 4 Today programme.
The prison reform campaigner said that the frequent moving of vulnerable inmates between prisons increased the likelihood that they would attempt suicide.
"If you are mentally ill or unstable or under stress, you are constantly faced with uncertainty - staff who don't know you, you're further from home, you've got no support systems and you feel like you are in some sort of hell," she said.
Pauline Campbell, whose daughter Sarah died in custody, told the BBC that prisons were being "overwhelmed" by high numbers of vulnerable people who needed care, not punishment
"They're being used as social dustbins for people who are mentally ill, drug and alcohol dependents, the homeless and so on," she said.
"And given that we have such a high proportion of prisoners who have psychiatric difficulties, it is inevitable that these tragic deaths will occur unless action is taken to prevent this happening."
The mother of the youngest child to die in custody in the UK accused the government of doing too little to protect vulnerable inmates.
Carol Pounder's son, Adam Rickwood, was 14 when he hanged himself with his shoelaces while on remand at the Hassockfield secure training centre in County Durham in 2004.
The teenager had been restrained with a controversial "nose distraction technique" which involved him being punched in the face.
She told Today: "The number of child deaths in custody is shocking. The government is failing to provide adequate support. They do not have enough properly qualified, fully trained prison officers in these children's institutions.
"They have nine weeks' training ... in an adult prison, the training course is a lot longer."
The prisons minister David Hanson said a review of the restraint of children in custody was due to publish its findings in about six months.
He said that while the number of self-inflicted deaths had gone up, the proportion of the prison population committing suicide was dropping. But, he added, there was no room for complacency.