The Secret Service is experiencing so severe a decline in the quality and quantity of intelligence information that its ability to protect the president and other public figures has been seriously impaired, according to Director H. Stuart Knight.

Intelligence information about potentially dangerous individuals and organizations is so bad in some cities, Knight said through a spokesman, that the Secret Service has advised President Carter, as it did his predecessor, Gerald Ford, not to visit those cities. Knight refused to name the cities or comment on whether either president had taken or ignored the agency's advice.

Knight first raised the problem of the quality of intelligence during a little-noticed session before a Senate Judictary subcomittee last summer, but he said last week there has been no improvement in the situation since. The agency is getting less than half the intelligence it did five years ago, he said, and the quality of that information has fallen sharply, for an overall decline of about 75 percent.

Part of the problem, Knight told the subcommittee, is a pair of laws: the 1974 Privacy Act which limits the information federal agencies can disclose from their files about individuals, and the Freedom of Information Art, which allows individuals to see their files to find out what kind of material is being kept about them and how it is being gathered.

There is also a psychological problem, Knight said, which affects law enforcement and intelligence agencies from the FBI to a local sheriff. "We have to look at the atmosphere in which these people now have to operate in terms of guidelines that may be imposed upon them and the attitudes of the various organizations to which they report . . .," he said. "(There is) a reaction - overreaction in my opinion, but a reaction nevertheless - to some of the alleged misuses of intelligence information in the past."

The alternative to getting good intelligence, Knight said, is adding manpower. "With the paucity of information we are receiving, we are only left with one alternative, and it is a poor alternative, at that. We really do not know what might occur. We feel that we must increase the number of people traveling with the protectee. That is a very, very poor second." The White House had no immediate comment on Knight's statements.

The Secret Service currently guards 18 people full-time: Carter and his family and Vice President Mondale and his family. Agents also are assigned round-the-clock protection duties for all presidential candidates and their families. In 1976 there were 15 candidates. The agency also is responsible for protecting visiting foreign dignitaries, who number close to 90 a year. The Executive Protective Service, the uniformed branch of the Secret Service, provides 24-hour, onsite protection for the more than 200 foreign embassies and chanceries in Washington. All of those protection duties. Knight said, have been jeopardized by the intelligence breakdown.