Prosecutors in the United States say data belonging to users of Megaupload 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 and shut down on 19 January.
US agencies are seeking to extradite founder Kim Dotcom and three other defendants from New Zealand. They were arrested on 20 January at the request of the FBI.
The BBC reports the deletion warning was contained in a letter filed by the US Attorney's Office.
Prosecutors said that the data which might be deleted was being held by the storage companies Carpathia Hosting and Cogent Communications Group.
''It is our understanding that the hosting companies may begin deleting the contents of the servers beginning as early as 2 February,'' it said.
The letter explains that in its investigations the US ''copied selected data'' but did not remove any servers from the premises of either company.
It goes on to note that the data "remains at the premises controlled by, and currently under the control of Carpathia and Cogent.
''Should the defendants wish to obtain independent access ... that issue must be resolved directly with Cogent or Carpathia.''
Megaupload lawyer Ira Rothken told the Associated Press that at least 50 million users have data which could be deleted.
He says because the company's funds are frozen, it is unable to pay those storing its data.
Megaupload was indicted in a federal court in Alexandria, Virginia, USA, on 19 January and the site was then shut down. The BBC reports it had about 150 million registered users.
An Associated Press business writer Josh Freed told Morning Report that users from all over the world, including New Zealand, will be affected.
He said users rely on Megaupload to store all kinds of data including family photos and personal documents.