Maxine Carr's release will go ahead as planned despite the theft of key papers, the prisons' boss has said.
Maxine Carr is expected to be released on Friday
Scotland Yard is investigating after documents containing details about her release from jail were stolen from a Home Office official's car.
But prisons' chief Martin Narey said the plans were still on track.
Carr, 27, will be freed on Friday after serving a three-year term for conspiring to pervert the course of justice with Soham killer Ian Huntley.
The Home Office has gone to great lengths to keep her release arrangements secret because of fears of vigilante action.
Carr has already been moved from Foston Hall Prison near Derby to a secure location, ahead of her release.
But she will have to stay in the country and report to probation officers regularly. Carr was given a three-year community rehabilitation order earlier this week after pleading guilty to 20 charges of benefit fraud and lying on job applications.
The stolen papers containing details of her release were later recovered on London's Hampstead Heath.
Mr Narey told Radio 4's Today programme it was "disappointing" the documents had been lost, but denied it had been "a dreadful business".
"Meticulous arrangements have been made for Miss Carr's release, they've gone ahead exactly as planned," he said.
"This was an accident we regret but it was made by a hugely-dedicated and able official who shouldn't have left documents in the back of a car.
"But nothing was in those documents relating to Miss Carr's new identity and the police believe this was a random theft.
"The important thing now is Miss Carr has a new sentence, the probation service has to supervise her, make sure she doesn't re-offend and hopefully is resettled back into society."
But Mark Oaten, Liberal Democrat home affairs spokesman, said the security lapse was "breathtaking".
"It's extraordinary that documents of this nature should be left in a car.
"I would have thought that documents like this should remain in a secure place in the Home Office," he said.
Opposition leader Michael Howard said the incident was one of "quite extraordinary incompetence" and called for a Home Office investigation.
Harry Fletcher, of the National Association of Probation Officers, said a great deal of time and expense would have gone into creating a new identity for her and preparing for her release.
The theft was a "severe embarrassment", he said.
Carr is believed to want to move to Humberside to be near her mother.
Meanwhile, Chief Constable David Westwood has reacted to criticism of Humberside Police.
They failed to keep records of a string of
sex allegations made in Grimsby that should have prevented Huntley securing a job as caretaker at Soham Village College.
"It could have happened to any police force in the country," Chief Constable Westwood told the Yorkshire Post newspaper.
"I don't get things right all the time," he added.
"Nobody can get everything right all the