The Justice Department said yesterday it will no longer use its powers under the Freedom of Information Act to block requests for government documents if the information in the documents was obtained illegally.

Although the act is intended to give the public access to documents held by federal agencies, it allows the government to exclude certain categories of information from disclosure.

The announcement yesterday referred to a provision in the act authorizing the rejection of requests for information whose disclosure might interfere with law enforcement activities.

However, a memo by Deput Attorney General Peter F. Flaherty, made public yesterday, instructed Justice Department attorneys that this provision "should not be used to conceal unlawful activities, regardless of the intent with which those activities were conducted."

Quinlan J. Shea Jr., who heads the department's office dealing with Freedom of Information questions, said Flaherty's directive means that the "law enforcement exemption" can no longer be used to shield information obtained by such means as illegal wiretaps or unauthorized searches.

Shea said this policy will probably lead to some increase in the amount of information released by the government without resort to a legal struggle. He added, though, that since the Justice Department has put tight restraints on the use of illegal investigative techniques by federal police agencies, the instances where such questions occur in FOI request are likely to become increasingly infrequent.

In addition, he cautioned, a determination that information has been obtained illegally does not mean that it will be released automatically.He said the department will still have to ensure that it doesn't fall into other categories that the act exempts from disclosure.

Although Flaherty's memo technically applies only to the Justice Department and those agencies like the FBI under its control, its effects should be felt throughout the government. The department could refuse to defend any other federal agency that is sued under the act if it fails to comply with the policy directive.