Zahid Mubarek was serving three months for theft

Zahid, 19, was battered to death by his cellmate at Feltham Young Offenders' Institution in London in March 2000.

Martin Narey told the probe into Zahid's death that he stayed on in the job after encouragement from the then home secretary, Jack Straw.

Mr Narey also said the level of care at Feltham was "woefully inadequate".

In a written statement to the inquiry, Mr Narey said: "I consider Zahid Mubarek's death to have been preventable, and, as the director general at the time of his death, I considered and discussed my resignation with the home secretary.

The care provided at Feltham for young people was woefully inadequate Martin Narey former Prison Service chief

"With Jack Straw's encouragement, I decided to remain in the post and continue with the programme of change that I believe has made a real difference to the experience of a large number of prisoners during the last five years."

Mr Narey was director general of the Prison Service from 1999 to 2003.

He told the inquiry that the Feltham institution was one of the "most difficult challenges" he faced during that time.

Mr Narey now heads the National Offender Management Service

"It had a history as a failing prison, its design was poor and the prison was having real difficulties in recruiting enough members of staff to deal with a very high rate of turnover of young prisoners.

"I was in no doubt at all that the care provided at Feltham for young people was woefully inadequate, and that the regime fell far short of my aspirations for decent prisons."

Zahid was beaten to death by Robert Stewart, now 24, with a table leg. He was hours away from his release for a first-time offence at the time of the attack.

Stewart, who was described as a psychopath at his trial, was jailed for life in October 2001 for the murder.

Mr Narey said that at the time the teenager was killed, 90% of new prisoners were displaying some form of mental disorder, such as alcohol or drug addiction or psychosis.

Robert Stewart: Serving a life sentence for the killing

He said "thousands" of current prisoners could be described as being as dangerous as Stewart.

He also said Feltham had the most "difficult and unreasonable branch" of the staff union, the Prison Officers Association (POA).

He told the inquiry he moved the chairman of Feltham's POA branch, but later apologised to him for the way the transfer had been handled.

He added: "During my time as director general, the incidents, repeat incidents, of self-inflicted deaths was 100 times greater than those perpetrated by one prisoner on another prisoner.

"It was a crisis. It remains a crisis."