The Justice Department said yesterday that Congress should expand the waiting period for gun sales to five days so the FBI can prevent lapses that have allowed almost 1,700 illegal firearms purchases since November.

Deputy Attorney General Eric H. Holder Jr. said the FBI cannot prevent all illegal sales because it is unable to complete background checks for every gun buyer within the three-day period mandated by Congress.

"The problem is not a function of FBI performance but is a function of the time allowed to make the background checks," Holder said. "It would be better if they gave us two more days. Each day they could give us would enable us to stop more [criminals] from getting guns."

Almost 1,700 convicted felons and others barred from owning firearms have bought guns since November because of failings in the background check system overseen by the FBI.

The "instant-check" system, which went into operation Nov. 30 after five years of preparation, responds to background inquiries from gun dealers with approvals within hours in 95 percent of sales, Justice officials said. But if there is no response within the stipulated three days, dealers are free to turn weapons over to the purchasers.

Justice officials said the instant-check system is hamstrung by a lack of computerized criminal records that may take years to overcome. They said that in some instances, the FBI's databases show only that a person was charged with a crime--which does not necessarily bar someone from owning a gun--and do not indicate whether there was a conviction. In those cases, the FBI must contact individual states directly to sort things out, a process that can take several days.

Congress has appropriated more than $200 million for states to computerize their criminal records. But Justice officials said there is no swift remedy.

"The fact is, when you're dealing with a system that is so old . . . it is going to take us a while to achieve a more perfect situation," said Bea L. Witzleben, a Justice Department lawyer who oversees gun issues. "It's going to take a long time to get there."

The Clinton administration's proposal to extend the waiting period to five days has run into opposition in Congress, where some lawmakers have countered with proposals that the waiting period should be shortened in some circumstances.

Congress held a fierce debate this spring over whether to require firearms sold at gun shows to be subject to criminal background checks. While Justice officials argued for a five-day waiting period for gun show purchases, one House bill would cut the time to 24 hours.

Justice officials said an internal study showed that 17,000 felons and others prohibited from owning firearms would have been able to buy guns if the waiting period had been 24 hours during the first six months that the instant-check system was in effect.

"The attempts to reduce the amounts of time are misguided and potentially could lead to tragic results," Holder said.

Sen. Frank R. Lautenberg (D-N.J.) said the disclosure in The Washington Post on Friday that 1,700 ineligible people have been able to buy guns since November "demonstrates that efforts to impose a 24-hour time limit on criminal background checks at gun shows are a mistake of monumental proportions."

The FBI said it has conducted more than 2.2 million background checks since the instant-check system went into effect. More than 47,000 gun applications have been denied.