WASHINGTON—The Justice Department said it provided the Supreme Court with erroneous information that helped it win a 2003 case upholding a blanket policy of denying bail to thousands of immigrants imprisoned while appealing deportation orders.

The department, in a Friday letter to the court, said it made “several significant errors” that greatly understated the time certain aliens with criminal records spend in no-bail detention. The 2003 opinion, Demore v. Kim, cited government data to hold that “the very limited time of...