A spate of prison suicides is linked to a rising number of inmates, the chief inspector of prisons has said.
Prison suicides hit record levels in 2004 and 2002
Ann Owers said the movement of inmates between prisons due to overcrowding was a cause of "increased anxiety".
Twelve prisoners have taken their own lives in as many days in England and Wales, the latest at Pentonville jail in north London on Wednesday.
The Home Office said prison deaths were "a tragedy" but clusters of suicides were unfortunately not unusual.
There were 95 prison suicides in England and Wales in the whole of 2004 - equalling the highest number, which occurred in 2002.
Ms Owers told BBC News there were so many new prisoners arriving at jails it was hard for staff to assess those who might be at risk from suicide or self harm.
She said the prison population was rising because more people were being sent to prison and being given longer sentences.
Many inmates were mentally ill, or had substance abuse problems, which Ms Owers said created a "volatile mixture" that could lead to suicide or self harm.
A Home Office spokeswoman said the level of self-inflicted deaths in April and May 2005 had been at its lowest since 1992.
She said tackling suicide and self-harm was a key priority for ministers and the Prison Service.
The spokeswoman added the Home Office did not know what had caused the recent spate of deaths but would try to find out and learn any lessons it could.
Ms Owers raised the topic at a briefing about her recent US visit, which looked at prison inspections.
She said there was a "groundswell" of support in America for an independent inspection system of prisons.
It follows allegations of ill-treatment by American guards at Abu Ghraib prison in Iraq, and Guantanamo Bay, Cuba.
In September, Ms Owers will take a British prison inspection team to Canada to inspect two women's jails criticised by a commission on human rights.
She said it was "ironic" other countries were turning towards the specialised expertise offered by British inspectors at a time when plans were afoot to merge criminal justice inspectorates into one agency.