Immigrants living in the United States unlawfully will have to leave the country to apply for resident “green cards” unless Congress extends a provision that expires next week.

Currently, undocumented immigrants who are eligible to apply for legal residence are allowed to remain in the United States while completing their paperwork - provided they pay a $1,000 fine.

The impending expiration of that provision has caused alarm in immigrant communities. And the Immigration and Naturalization Service is expecting a wave of applications from people rushing to beat the clock.

“The message is to get that application in before Sept. 30,” Paul Virtue, the INS’ acting executive associate commissioner, said Monday.

In 1994, Congress wrote a regulation allowing undocumented immigrants to remain here while applying for permanent residence provided they pay a fine and either be already on track for legal residence or related to a U.S. citizen or legal permanent resident.

The idea for the proposal was twofold: Reduce the hardship on foreigners eligible to legalize their status and lessen the pressure on U.S. consulates overseas where immigrants previously applied.

In 1995 and 1996, some 345,000 people took advantage of the rule, which quickly provided a new revenue stream for INS. This year, the agency will take in nearly $200 million in fines.

The Clinton administration has asked that this provision be extended, but Congress has yet to decide the issue.

The Senate, in the bill that funds INS operations, approved an extension. The House, due to take up its bill later this week, doesn’t have similar language - meaning House and Senate negotiators will have to resolve the issue later this fall when they iron out differences in their respective bills.