Millions of federal employes soon will receive a toughly worded memorandum from President Reagan reminding them that unauthorized disclosure of classified information is "improper, unethical, plain wrong" and "illegal," and warning that "appropriate administrative action" will be taken against anyone found to have made such a disclosure.

"Where circumstances warrant, cases will also be referred for criminal prosecution," the memo adds.

The memo is the lastest step in Reagan's continuing campaign to shut off leaks to the media of classified information dealing with diplomatic, military and intelligence activities. The two-page document was given to Cabinet officers Tuesday for distribution within their agencies.

Long-standing government directives forbid unauthorized disclosures by employes of agencies that normally deal with national security policy, such as the Department of Defense, CIA and the National Security Agency.

But administration officials said they could not recall any previous instance where a president had sent a personal memorandum dealing with a national security issue to all federal employes, who now total about 2.6 million.

"It is unusual, and it is intended to be," said one senior administration official.

In his memo, the president said he was doing this to "underscore . . . the seriousness" of the problem.

While most administrations have complained of media leaks, Reagan has made a special point of it, saying in February that he was "up to my keister" with leaks.

On March 11 the president signed a new executive order that significantly expanded previous directives. Among other things, it directed all persons in any agency with access to classified information to sign a non-disclosure agreement.

It also ordered a revision of existing regulations so that all federal employes could be required to take lie detector tests, and it approved "adverse consequences" as a minimum punishment for persons refusing such tests.

The March directive was challenged sharply by civil liberties groups and many journalists as an attempt to curtail freedom of information, intimidate the bureaucracy, stifle public debate on security policy and inhibit news reporting.

Many critics also argued that information frequently is classified for reasons having little to do with national security and that the government frequently leaks such information when it suits its purpose.

In July, the House and Senate both voted to prohibit the Pentagon temporarily from expanding use of lie detectors.

Administration officials made the memorandum available to The Washington Post yesterday and consented to be interviewed provided they were not be identified.

They portrayed the document as an attempt by the president to appeal directly to government employes' sense of responsibility and public trust.

"As public servants, we have no legitimate excuse for resorting to these unauthorized disclosures," the president said, arguing that "there are other means available to express ourselves," including the right to quit and speak out later.

(The March directive, however, requires all former bureaucrats to submit future writings based on their government work to pre-publication government security review.)

Officials say the Tuesday memo's personal appeal to duty is not really connected to the March 11 directive. The earlier document established government-wide machinery for legal obligations, investigative policy and regulation of contacts with the media. In his latest action, one official explained, Reagan "wanted to say something more, an expression of concern to each employe."

The administration has declined to cite publicly any instances where national security has been hurt by news leaks, contending that this would tend to confirm information or intelligence-gathering capabilities.

Officials claimed that the leaks have continued despite the March order and that this led to a decision to issue a personal memo. What is particularly bothersome, they said, is that some leaks have what one official described as a "gratuitous quality" to them, meaning that officials are providing not only information but details about how it was gathered, which injures U.S. intelligence operations.

Asked for examples, officials cited stories concerning recent policy decisions and activities in Central America, a New York Times account in April of a highly classified document dealing with Central America, a CBS-TV report about the bombing of the U.S. Embassy in Lebanon that was reportedly based on intercepted diplomatic communications, and a Boston Globe story about the movement of a U.S. aircraft carrier in the Mediterranean at a time when a coup might have been developing in Sudan.

Reagan's memo says Attorney General William French Smith has been asked to look into several recent disclosures. Officials would not be specific, but said they didn't think any probe had been completed.