An investigation is under way after a coroner left a file on the investigation into the murder of schoolgirl Alice Gross on a train.
Chinyere Inyama, the senior west London coroner, lost the 30-page police file in November last year, a month after her body was found in a river in the capital.
Police tried to recover the file, which contained evidence against prime suspect Arnis Zalkalns, but concluded it had probably been "destroyed as waste".
The Ministry of Justice is investigating the incident and why the file was ever taken from the coroner's office.
Alice disappeared last August from her home in Hanwell, west London, sparking Scotland Yard's biggest search operation since the July 7 London bombings.
The 14-year-old's body was found weighted down in the Grand Union Canal in Ealing, west London on September 30.
Days later Zalkalns, 41, was found hanged in Boston Manor Park, west London, and police later said the Latvian - who had previously served seven years for murdering his wife in his home country - would have been charged with Alice's murder if he had still been alive.
An inquest into Alice's death was opened and adjourned last October, but the coroner lost the vital document - given to him with preparations for the inquests into the deaths of Alice and Zalkalns - a month later.
A spokesman for the Metropolitan Police said: "In November 2014 the MPS was informed by HM Coroner, London West, that he had inadvertently disposed of a single document relating to the police evidence against Arnis Zalkalns.
"An investigation to recover it was undertaken. This concluded that it was highly likely it had been destroyed as waste."
Details of the evidence against Zalkalns were given to the media in January with the agreement of the Crown Prosecution Service and the coroner.
The Ministry of Justice said: "This clearly appears to be a troubling incident. A full investigation is now under way."
A full inquest into Alice's death is due to be held at the end of November or the beginning of December, and Mr Inyama reserved judgment on whether it will be a jury inquest.
He is expected to hand down his ruling on the scope of the inquest in the next week.