President Reagan moved yesterday to prevent leaks of classified government information, issuing an executive order requiring every federal employe with a security clearance to sign a nondisclosure pledge and submit to lie detector tests if asked.

The new order applies to hundreds of thousands of U.S. government workers.

Federal employes, with the exception of those in the CIA, the National Security Agency and certain sections of the Justice and Defense departments, currently have the right to refuse to submit to lie detector tests in the investigation of leaks without their refusal being held against them or included in their personnel files.

According to the new order, all federal employes who have access to classified material "may be required to submit to polygraph examinations, when appropriate, in the course of investigations of unauthorized disclosures of classified information." The order warns that an employe who refuses to take a polygraph test may be subject to "adverse consequences."

John Shattuck of the American Civil Liberties Union called the change a "very sweeping new authority to curtail freedom of information, an effort to put a substantial straitjacket on the press' treatment of information the administration claims relates to national security, and to use the most sweeping and intrusive investigatory technique in this area--a mandatory polygraph test."

Because they are not always considered to be reliable, lie detector tests are not allowed as evidence in federal courts.

The new order also would require all employes holding especially sensitive clearances--called Sensitive Compartmented Information clearances, dealing mainly with intelligence sources and methods--to get prior clearance for any publication that might include classified information. Now such clearance is formally required only by the CIA and the National Security Agency.

An administration official, briefing reporters at the Justice Department, said that the department has taken the position, based on the 1980 Supreme Court decision in the case of former CIA employe Frank Snepp, that all government employes with access to classified information have an obligation not to disclose it in books or articles, regardless of whether they have signed formal agreements.

Snepp was required by the court to forfeit profits on his book because he had not complied with a signed agreement that required him to submit it for a CIA review before publication.

The new order also requires federal departments and agencies to adopt "appropriate policies" to govern contact between the news media and government employes, "to reduce the opportunity for negligent or deliberate disclosures of classified information."

The policies were not spelled out in the Reagan order and will be left up to each agency. But the administration official said they could include limiting access to areas where classified material is kept, keeping logs of such contacts, and other unspecified measures.

The executive order provides that employes who refuse to cooperate in leak investigations be subject to mandatory punishment, including at least denial of future access to classified information, and even dismissal.

The order clarifies the FBI's authority to investigate leaks for various federal agencies as "potential violations of federal criminal law."

But the administration official, noting that there has never been a successful criminal prosecution of a leak of classified information, said, "We have decided, in a way, to lower our sights. For many years, the government has had a totally ineffective program of enforcing the laws against unauthorized disclosure of classified information.

"The reason is that the government has pursued the illusory goal of criminal convictions. This directive recognizes that milder sanctions . . . , if imposed successfully and more frequently, are better deterrents than theoretical but never achieved . . . criminal convictions," he said.

The administration official refused to identify the leaks that prompted the new policy, saying such information is classified.

The new policy comes at a time of continuing concern by the administration about leaks.

Last year, lie detector tests were given to a large group of officials at the Defense Department following published reports of a Pentagon wish list that would have added $75 billion to the department's budget.

Then, last month, after a series of leaks that involved White House aides, Reagan said he was "up to my keister" with the leaks to the news media.