A former information minister charged today that President John Vorster and the current finance minister knew about secret Information Ministry projects earlier than they have admitted, in effect saying the two officials publicly lied about involvement in the scandal.

If the charges by Cornelius Mulder, who was forced to resign for his role in the scandal, are substantiated by a report due out Saturday by government investigators appointed by Prime Minister Pieter W. Botha, it could result in the resignations of Vorster and Botha.

Botha has told Parliament he would resign and call an election if any member of his Cabinet was found to have known about the improper financing of the proapartheid newspaper, The Citizen, earlier than previously admitted. Vorster was prime minister at the time of the project.

In reaction to Mulder's statement, Botha said simply that "these matters" are being considered by his investigating team. This reaction to charges from Mulder, who has powerful backing from the ruling National Party's conservative wing, and who was Botha's principal rival in the premiership race last September, contrasts sharply with his rejection as "smear stories" of similar allegations over the past two weeks by his own top aide, Eschel Rhoodie.

Last December, Botha's probe team found Rhoodie, Mulder and the country's former intelligence chief, Hendrik van den Bergh, responsible for the secret financing of The Citizen, which was set up to counter negative press coverage of the government by the opposition press. But public rejection to these findings has raised suspicion that the conclusion of the investigators was a cover-up that had Botha's approval.

In the face of steadily dissolving credibility, Botha ordered his investigators to take another look at whether any of his ministers were involved in the newspaper plan and to report to him by March 31. If the commission "amends" its December conclusions to duplicate Vorster and Finance Minister Owen Horwood, Botha would almost certainly have to call new elections, analysts here say.

It is unlikely that an election, even under the cloud of scandal, would cause the Afrikaner-dominated National Party to lose its majority to the enfeebled opposition parties. Nevertheless, it could set off a power struggle within the party itself, which might force Botha out as party leader and allow an archconservative, Andries Treurnicht, to take over as prime minister. Such a development would cut off Botha's tentative moves to alter the country's racially discriminatory policy of apartheid.

The strains may also affect U.S.-led diplomatic efforts to halt Soviet expansion in southern Africa by getting a negotiated settlement to the guerrilla war in the South African-administered territory of Namibia, or Southwest Africa. Compromise solutions to the conflict may be rejected by Botha if his political instincts tell him his party needs an issue around which it can unify to offset divisions caused by the scandal.

Mulder's allegations today were a reply to an unusual public statement by Vorster last Thursday, when he said he only learned of the secret funding of the paper in August 1977, 11 months after it had been launched. Vorster also denied that Rhoodie and Mulder had regularly reported on their secret activities to any member of the Cabinet.

Mulder today alleged that he and Vorster discussed changing the format of The Citizen in Vorster's office in December 1976. The former minister also charged that the project had been decided on at a meeting Dec. 4, 1975, and one of those who attended, van den Bergh, had reported to Vorster on the same day.

Although Mulder says he does not know if van den Bergh told Vorster of the newspaper project on that day, he goes on to say: "Everyone is agreed that Mr. Vorster, knowing of the previously mentioned establishment of the newspaper, prohibited neither me nor the officials of the department who were involved from going aheadwith it."

He said Horwood "had knowledge before April 1978 of the fact that The Citizen was supported in a particular way by secret money coming from my department."

Horwood has stated that he learned of The Citizen's financing only in September of last year.

In closing, Mulder said, "I am sorry that I now have to publish these facts in this way... For almost a year I have kept silent in spite of all the accusations and allegations and hoped that the matter would clear itself up... South Africa deserves that this matter should now end and that the persons concerned should bury the hatchet and find a joint solution. It has lasted long enough."