A cover-up led to Ofsted escaping blame for its part in the Baby P scandal, according to a BBC documentary.
Baby P: The Untold Story hears from an unidentified Ofsted inspector.
Assessment documentation of Haringey's children's services department "disappeared", the inspector said.
Ofsted said it "has always been entirely open" about the "change between Haringey's provisional annual performance assessment grade and the judgment finally given in 2008".
Peter Connelly, known as Baby P, died at the age of 17 months in 2007 after being subjected to months of abuse.
He had more than 50 injuries, despite being on the at-risk register and receiving 60 visits from social workers, police and health professionals over eight months.
Peter's mother, Tracey Connelly, her boyfriend, Steven Barker, and his brother, Jason Owen, were jailed in May 2009 for causing or allowing the child's death.
The assessment documentation disappeared after Ofsted's grading of the council was changed from grade three, meaning "good", to grade one, indicating "inadequate", following publicity about Haringey's failings, according to the inspector.
"I don't know who made the decision to delete those files. But if you remove that information you remove your accountability," the whistleblower said.
"I felt that was a cover-up.
"I think there were senior people within Ofsted who have never been held accountable for Ofsted's behaviour around the Baby P case," said the inspector, whose voice was disguised to secure anonymity.
The 90-minute documentary also hears damning evidence about Great Ormond Street Hospital (GOSH), which managed the borough's medical staff.
Sharon Shoesmith, who was director of children's services at the council at the time, tells the documentary she and her colleagues "wept together" when they read about what Peter had suffered.
Ms Shoesmith was sacked in December 2008 by the then children's secretary Ed Balls.
She claimed she had been unfairly dismissed and the Court of Appeal ruled in her favour in 2011, saying she had been "unfairly scapegoated"
She was awarded £679,452 following her unfair dismissal claim.
"The child was so vulnerable and we missed it," Ms Shoesmith said in the documentary. "All of us - the police, the social workers, the health people, all of those health agencies - you could just go through it again and again. If only this, if only that.
"We missed it and we missed it and we missed it."
A spokeswoman for the education watchdog said: "Ofsted has always been entirely open about the fact that there was a change between Haringey's provisional annual performance assessment grade and the judgment finally given in 2008.
"The reasons, as established at the time, were clear. In the first instance data provided by Haringey to contribute towards the provisional assessment was found to be misleading - a fact established by the emergency inspection of November 2008.
"Further, as would be expected, the evidence of serious safeguarding concerns uncovered by the emergency inspection itself - which took place between the provisional grade being given and the final assessment - was taken into account in the final grading."