The way the public is protected from criminals released from prisons has been strongly criticised in a report by three criminal justice watchdogs.
The report acknowledged "effective procedures already in place"
The public risk posed by offenders had not been analysed when they left prison in 40% of cases, it found.
There were sometimes long delays in finishing work after release, it added.
But the Home Office said assessment of high and very high risk offenders had increased to 91% since the report was compiled late in 2005.
The report was written by prisons watchdog Anne Owers, probation chief Andrew Bridges and Chief Inspector of Constabulary Sir Ronnie Flanagan.
It was commissioned after the Probation Service was criticised for failing to keep a proper check on freed offenders who then went on to commit serious crimes including murder.
These include Anthony Rice who killed mother-of-one Naomi Bryant, nine months after he was freed from a 16-year jail sentence.
Another case involved the murder of John Monckton - the financier stabbed to death at his Chelsea home in November 2004 - who was killed by Damien Hanson, then on probation.
The report found that police were not always told when inmates were let out, meaning officers "had no input into these temporary conditions".
"In a fifth of cases of prisoners just starting their sentence, and just over a third of those prisoners about to be released, we found little evidence of positive, proactive and timely work between prisons, probation and police," it added.
And there were shortcomings in analysing risks posed by some of the UK's most violent offenders, the report said.
"In only half of the relevant probation cases had a comprehensive risk management plan been completed on high and very high risk of harm offenders within five working days of their release from prison," it said.
In a joint statement, the report's authors said there had been "many encouraging examples of effective work".
But there was "a clear need for improvement" in about one-third of cases looked at, they added.
"While it will never be possible to eliminate risk when an offender is being managed in the community, it is right to expect the work to be done to a consistently high standard."
Rice killed a woman nine months after he was freed
The mother of Naomi Bryant said the system had to change.
"If you let these people out then they're supposed to be monitored," Verna Bryant told BBC Five Live.
"When [Anthony Rice] disappeared all night long nobody knew he'd even gone.
"They say there's too much paper work and not enough people to monitor it but then they should make more prisons and employ more people."
A Home Office spokeswoman said the government's highest priority in supervising offenders was public protection.
"This report acknowledges the effective procedures that are already in place," she said.
"Since the inspection concluded a year ago the completion of initial risk of harm assessments, risk management and sentencing plans on high risk offenders have significantly increased, exceeding targets set by the Home Office.
"Offenders in custody and the community are managed accordingly through risk of harm procedures with resources concentrated on those who present the highest risk."
Inspections for the report took place in late 2005 in eight probation areas - County Durham, Derbyshire, Hampshire, Kent, Lancashire, Newham in London, North Wales and Suffolk.
Nine prisons were also inspected for the report. They were Acklington, Altcourse, Canterbury, Durham, Elmley, Frankland, Maidstone, Wandsworth and Wymott.