A new federal policy that could require thousands of immigrants to return to their home countries before petitioning for green cards has applicants and immigration lawyers scrambling to to understand how the change will affect the path to permanent residence.

On Friday, U.S. Citizenship and Immigration Services, the agency that oversees the green card system, said that only in “extraordinary circumstances” would people already in the United States be granted permanent residence. Otherwise, anyone seeking permanent U.S. residence would need to apply at an American consulate in their home country.

Immigration lawyers and advocacy organizations said over the weekend that the change would reduce green card applications. In 2024, 1.4 million green cards were issued, and more than 800,000 of the recipients were already in the United States and had their immigration status modified as part of the process.

The change announced on Friday will be of particular concern to those who are married to U.S. citizens and seeking permanent residence, said Charles Kuck, an immigration lawyer and the former president of the American Immigration Lawyers Association. Such immigrants typically need to resolve their own immigration status before seeking a green card and they have generally been able sort out those issues while remaining in the United States.

“This is simply an attempt to slow immigration,” Mr. Kuck said, “and make immigration so unpleasant that you go home.”

Zach Kahler, a spokesman for the agency, said on Friday that the policy would remove a loophole where immigrants “slip into the shadows and remain in the U.S. illegally after being denied residency.”

The memo notes that allowing immigrants to remain in the country while their green card applications are processed, “is a matter of discretion and administrative grace.”

Efrén Olivares, the vice president of litigation and legal strategy at the National Immigration Law Center, said the new policy is about disrupting the green card process.

“It’s going to upend people’s lives in every sense of the word,” he said.

Most applicants have been allowed to remain in the United States with their families, which can include spouses and children who are U.S. citizens, while the green card process plays out. But now remaining in the country, may be the exception. In the past, Mr. Olivares said, someone may have been asked to apply from their home country in extraordinary circumstances, like if they had been deported several times or if they had a serious criminal history.

Approval for permanent residency through a U.S. citizen relative can vary depending on the relationship, Mr. Olivares said. For a spouse, the waiting period could be about a year; for siblings it can take upward of five years; and for parents, it could take as long as a decade.

Several immigrants, who spoke on the condition of anonymity for fear of retaliation, said on Saturday that they were confused and worried about what this change could mean for their own applications or those of their partners.

After the announcement, an Albanian woman with a green card said she called her fiancé, who lives in Italy, trying to figure out what the change meant for them. The woman received a green card in 2022 through the lottery system, but her fiancé is expected to come to the United States this year on a work visa. She said they are now hiring an immigration lawyer to help figure out how to proceed.

The impact of this change may not be immediately visible, but will be strongly felt, said Karla Ostolaza, the managing director of the immigration practice at the Bronx Defenders, a public defense nonprofit.

“They will stay in the shadows, remain without status and vulnerable to exploitation, and fence off any better opportunities to obtain status, even if they’re completely eligible,” she said.

Maia Coleman and Oishika Neogi contributed reporting.