Attorney General Jeff Sessions issued a directive on Thursday that places limits on a tool commonly used by immigration judges and could put hundreds of thousands of deportation cases that are essentially closed back on federal court dockets.

The move, issued in an interim decision, is unlikely to reopen all the cases. But Mr. Sessions said that immigration courts could not put such cases on indefinite hold by using a practice known as administrative closure, which temporarily removes a case from a judge’s calendar and delays a proceeding that could remove an immigrant from the country.

Immigration judges “do not have the general authority to suspend indefinitely immigration proceedings by administrative closure,” he wrote in the decision, and the practice “effectively resulted in illegal aliens remaining indefinitely in the United States without any formal legal status.”

The move injects fresh uncertainty into the lives of undocumented immigrants living in the United States, and some critics say the decision could lead to their deportation before they could gain legal status.