President Clinton has decided to permanently ban a practice that has allowed the importation of popular semi-automatic assault weapons like the AK-47 and the Uzi, closing what he regards as a major loophole in the nation's gun control laws, administration officials said.

As many as 1.6 million rifles for which permits are pending or have already been issued would be blocked from entering the country by the executive order Clinton is scheduled to announce at a White House ceremony Monday, the officials said.The president will declare that 59 models of military-style riles previously certified for import because they had been modified to be used for sporting purposes will now fall under laws and regulations banning assault weapons.

Although the measure has been widely anticipated since Clinton imposed a temporary ban on the modified rifles in November, both administration officials and opponents of the ban depicted it as the most rigorous gun control measure Clinton has imposed by executive action.

"The issue has been studied and considered, and now the president is moving forward to ensure that these deadly weapons do not end up on our streets," said a senior White House official.

"No firearms in America are protected now that the president himself is deciding which guns will be banned regardless of what laws Congress has passed," said Tanya Metaksa, chief lobbyist for the National Rifle Association.

Major restrictions on the importation of foreign-made assault weapons were first imposed in 1989 following a schoolyard shooting in Stockton, Calif. In 1994, Congress enacted a broad-er prohibition on the manufacture, sale and possession of most assault weapons that included a ban on the production of large-capacity magazines that can hold 10 or more rounds of ammunition.

Foreign gun manufacturers responded by creating modified versions of the prohibited weapons that did not include military-style features such as flash supressors, pistol grips and folding stocks that would cause them to be categorized as unlawful assault rifles.

Weapons dealers imported the modified rifles under a provision of a 1968 gun control law that allows foreign-made rifles to be sold here only if they are clearly meant to be used for hunting, target practice or other sporting purposes. Since 1991, more than 425,000 of these modified rifles have been imported, according to the Bureau of Alcohol, Tobacco and Firearms.

At the prompting of Sen. Dianne Feinstein, D-Calif., Clinton suspended the import of the modified rifles last November and asked the bureau to determine whether the guns were being used for legitimate sports.

The president signaled his point of view in his weekly radio address Nov. 15, saying, "as effective as the assault weapons ban has been, we know that some foreign gun manufacturers are getting around the ban by making minor modifications to their weapons that amount to nothing more than cosmetic surgery."

The bureau contacted hunting guides and firing ranges to determine the sporting uses of the guns and convened technical experts to examine the weapons. According to a senior official at the Treasury Department, which oversees the bureau, the study concluded the 59 models of modified guns that will be subject to the new import ban do not fall into the sporting category primarily because they can accept large capacity magazines.

The gun-by-gun approach taken by the administration has drawn criticism from advocates of more aggressive action such as the Violence Policy Center, which favors a comprehensive rule that treats assault weapons overall as a specific class of firearm.

In a newsletter last week, the NRA warned that the new ban on imports "demonstrates once again that no matter what they say, the goal of Bill Clinton and his minions in Congress is a complete and total gun ban."