US prosecutors have said that data belonging to Megaupload users and stored by third parties could be deleted as soon as Thursday.
Users have been unable to access data since the file-sharing service was raided.
The warning was made in a letter filed by the US Attorney's Office, the Associated Press news agency reported.
Megaupload's lawyer Ira Rothken told the agency that at least 50 million users had data which could be deleted.
Mr Rothken said that freezing of Megaupload's funds meant it was unable to pay those who were storing its data.
In the letter prosecutors said that the data which might be deleted was being held by the storage companies Carpathia Hosting and Cogent Communications Group.
Neither they nor the US Attorney's Office have responded to emails from the BBC.
Mr Rothken told the agency that he was "cautiously optimistic" that a deal could be done to save the data from being wiped.
He said that the data would be needed by the defence.Legitimate data
Megaupload was shut down on 19 January.
It had about 150 million registered users, making it one of the most popular file-sharing services in the world.
US authorities are seeking to extradite founder Kim Dotcom, also known as Kim Schmitz, and three other defendants from New Zealand to the US.
Prosecutors have accused it of costing copyright holders more than $500m (£320m) in lost revenue.
But a number of users have said that they have been unable to access legitimately uploaded material as a result of the legal action.
After the shutdown one user tweeted, "I'm vehemently against copyright infringement: the files I lost were created & owned by me for my job."