The Reagan administration is to publish today its latest, and what it says is its last, modification to a controversial secrecy pledge required of civilian and military personnel with access to classified information.

The secrecy agreement, which already has been signed by an estimated 2 million persons in 67 agencies since the administration began using it in January, has been criticized by members of Congress and some government employes who believe it is intended to stifle the flow of information from the executive branch.

The form requires the employe to pledge not to disclose either "classified" or "classifiable" information. It is the use of the word "classifiable" -- which critics say is vague -- that has people up in arms.

For example, Sen. Charles E. Grassley (R-Iowa) said in an interview that the term "classifiable" could be used by the administration against any employe it wanted to "go after."

"Classifiable could mean anything. It will have a chilling effect on those working for government who will not disclose anything for fear that at a later a date it might turn out to have been classified," Grassley said.

The form clearly states that employes who refuse to sign it will lose their security clearance and probably their jobs.

The administration will publish in the Federal Register today a clarification, its second, of what "classifiable" means.

The agreement has been condemned by senior figures in the House and Senate as a reversal of constitutional rights and whistle blower protection laws.

Rep. John D. Dingell (D-Mich.), chairman of the House Energy and Commerce Committee and its oversight subcommittee, has warned that congressional opponents are prepared to legislate or "use the purse" to block the measure in its current form.

"It is obviously designed to suppress information on waste, fraud and abuse and to deny Congress the power to do its business," Dingell said. He added that it was "no coincidence" that the new determination to implement the measure coincided with the Iran-contra hearings.

The administration faces an imminent clash with a senior Air Force official, A. Ernest Fitzgerald, who has made whistle blowing on government waste a cause celebre. Fitzgerald, whose job as a management systems deputy at the Air Force involves investigating fraud and abuse, said he will not sign the agreement without major changes. He has until Aug. 21 to do so.

Fitzgerald said Air Force personnel, out of fear of losing their jobs, are signing the agreement without understanding the implications. "I, for one, do not want to be set up for a frame," he said.

At the center of the row is form SF 189, which springs from a controversial National Security Decision Directive issued by the Reagan administration in 1983 that authorized polygraph testing and required pre-publication reviews.

The testing and reviews were shelved, and it was widely believed that the nondisclosure agreements had also been abandoned. Then form SF 189 started circulating early this year.

It was drawn up by the Information Security Oversight Office (ISOO), a little-known arm of the General Services Administration that has been strengthened under the Reagan administration and takes its policy guidance from the National Security Council.

Because of the pressure, ISOO director Steven Garfinkel has already made adjustments to the wording of the form and has offered two clarifications of the meaning of "classifiable," the latest of which is to be published today.

The form states that employes should not disclose information that "is either classified or classifiable" under the standards of executive orders or statutes "that prohibit the unauthorized disclosure of information in the interests of national security."

On Aug. 3, the office said "classifiable" referred to information that should have been classified "but which, as a result of negligence, time constraints, error, lack of opportunity or oversight, has not been marked as classified information."

It said, "A party to SF 189 would violate its nondisclosure provisions only if he or she disclosed without authorization classified information or information that he or she knew, or reasonably should have known, was classified, although it did not yet include required classification markings."

The office is to rule today that the "term classifiable does not include any information that is not otherwise required by statute or executive order to be protected from unauthorized disclosure in the interest of national security."

The form's critics are not appeased by the clarification and want the term "classifiable" removed. Garfinkel said every effort has been made to answer criticisms, and no more changes are planned.

Rep. Barbara Boxer (D-Calif.) said: "The form is so vaguely worded that people will be afraid to blow the whistle on anything."