The sensitive material was found in a storage facility also being used as a cannabis farm in east London, according to shadow attorney general Emily Thornberry.
She said she was "profoundly shocked" the confidential details of witnesses had been lost by the Serious Fraud Office (SFO).
In a written parliamentary answer to Ms Thornberry, solicitor general Oliver Heald said the witnesses' identities were among the 32,000 documents, 81 audio tapes and electronic media from 59 sources the SFO accidentally sent to the wrong owner.
In a separate answer, he said the SFO has spent more than £10,000 finding the files and investigating their disappearance.
Officials are in the process of contacting all of those affected and have ensured the same mistake will not happen again, an SFO spokeswoman said.
Last month, the SFO said 98% of the material, which was part of its discontinued investigation into allegations concerning BAE Systems' international dealings, had been found. After halting the investigation, one of the suppliers of evidence had asked for it to be returned and was sent extra documents by mistake.
A spokeswoman said: "The SFO has been taking this matter very seriously since identifying the loss."