Attorney General Jeff Sessions barred the nation’s immigration judges Thursday from putting deportation cases on hold, a practice used in hundreds of thousands of cases of immigrants who needed time to gain legal status or were found to be low priorities for removal.

The procedure known as “administrative closure,” used by immigration judges since the 1980s, “lacks a valid legal foundation,” Sessions said in a decision based on his authority over immigration courts, a branch of his Justice Department.

Lawyers and advocates for immigrants said they feared the decision would lead to many deportations of migrants who would no longer have the opportunity to establish their right to remain in the country, while adding huge numbers of cases to the already overwhelmed immigration courts.

Sessions is “making it more complicated for people who have legitimate reasons to become legal,” said Jeffrey Chase, a New York immigration lawyer and former immigration judge. In many cases, he said, “the end result is going to be quick deportation.”

Trina Realmuto, an attorney with the American Immigration Council, said the decision was “harmful to immigrants and their communities” and reflected Sessions’ “anti-immigrant animus.”

Immigration courts decide whether immigrants who enter the U.S. without authorization, or overstay their visas, have legal grounds to remain in the country, such as political asylum or undue hardship to U.S. family members, or must be deported. Sessions has ordered every immigration judge to complete 700 cases a year, a sharp increase in their current workload, and has separately called for criminal prosecution of all illegal entrants.

He has also taken over several cases from the immigration courts to decide for himself, including the subject of Thursday’s decision. In another such case, he ended immigration judges’ previous obligation to hold a full hearing for anyone seeking political asylum. Sessions is also considering eliminating the right to asylum for victims of domestic violence and other nongovernmental crimes in their homelands. Any of those decisions can be challenged in federal appeals courts.

The procedure Sessions prohibited Thursday has been used by past administrations to allow immigration courts to manage their dockets, decide which deportation to hear first, and give some immigrants time to change their status.

For example, an immigrant might be seeking a visa through marriage to a U.S. citizen or appealing a criminal conviction that was grounds for deportation. The immigration courts already have a backlog of nearly 700,000 cases, a number that would rise to over a million if administrative closures are eliminated, said Sara Ramey, executive director of the Migrant Center for Human Rights.

But Sessions said the closures, purportedly temporary, are “effectively permanent in most instances,” leaving cases suspended without a final resolution.

He said immigration judges need not return all the closed cases to their calendars immediately, but could phase them in.

Chase, the former immigration judge, said the ruling might be appealed on the grounds that Congress has given immigration judges legal authority to manage their dockets. Bill Hing, a University of San Francisco immigration law professor, was less hopeful, saying Congress has largely left the management of those courts to the attorney general.

San Francisco Chronicle staff writer Hamed Aleaziz contributed to this report.

Bob Egelko is a San Francisco Chronicle staff writer. Email: begelko@sfchronicle.com Twitter: @BobEgelko