One of the country's leading probation officials has resigned ahead of a critical report on dangerous criminals who have re-offended after their release from prison.

David Scott, director of the London probation service, sent an email to colleagues saying he was leaving his post early after an investigation revealed serious failings in his department.

The report focuses on an individual case, which cannot be reported for legal reasons, but found a series of failings in the system to supervise prisoners after their release.

The most serious lapse of this kind resulted in the murder of John Monckton, a City banker who was stabbed to death on the doorstep of his Chelsea home by two former criminals.

Mr Scott told his colleagues: "It had been my intention to retire on my 60th birthday at the end of July.

"In light of the failures within London probation identified in a recent report I believe the right course of action is to step down."

The confidential report says errors were "a consequence of high caseloads, relative inexperience, and . . . insufficient supervision".

Harry Fletcher, assistant secretary general of Napo, the probation officers' union, said the system was in "meltdown".

At least half of trainee probation officers due to qualify this autumn have already been told there will be no permanent jobs for them.