Once again, a prominent opponent of South Africa's system of racial segregation has found his private life used by the government to discredit his views.

The target this time is Allan Boesak, a prominent church leader and antiapartheid campaigner, who confessed at a public meeting in Cape Town Saturday that evidence gathered by the police and published in the country's biggest daily newspaper that he was having an extramarital affair was true.

The disclosure seems likely to damage Boesak, both because of the implications of infidelity by a church leader -- he is married with four children -- and because he had initially denied the report.

Boesak holds the title of actuary, the second most senior position, in the mixed-race branch of the Dutch Reformed Church. A church commission is studying the implications of the disclosure. It could decide to suspend him as a minister.

If it does, Boesak could be compelled to resign as president of the World Council of Reformed Churches, a position to which he was elected three years ago, giving him an international status that added to his influence as an antiapartheid campaigner.

Such an outcome would be greeted with delight by the South African government and its predominantly white Afrikaner supporters, to whom Boesak has become a thorn in the side.

He was a key figure in having apartheid declared a heresy at the last conference of the world council in 1982, resulting in the suspension from the council of the white Dutch Reformed Churches to which most government supporters belong.

Boesak has also used his international status to campaign abroad, including the United States, for increased foreign pressure on South Africa to try to force it to abandon apartheid.

But while many whites are pleased at seeing Boesak in trouble, there is some unease at the police methods that have been revealed and the role of the press in publicizing their undercover findings.

Most South African newspaper editors recount occasions when they have been offered "exclusive" information about the private lives of prominent government opponents that could only have been gathered by a security police investigation. Usually only the more committed progovernment organs have published such material.

This time it was different. The news that Boesak was having an affair with a young white church worker was published last month by The Johannesburg Star, the country's biggest daily and one of its most reputable newspapers.

The thrust of The Star's report was that Boesak was the victim of a "dirty tricks" campaign by the security police. The newspaper reported that "smear pamphlets" and tape recordings implicating Boesak were being distributed, and claimed to have evidence that two security police colonels were involved both in spying on the clergyman and in distributing the pamphlets.

At the same time The Star reported that its inquiries into the "smear campaign" had revealed that Boesak was indeed having an affair with the church worker.

This has led to a major controversy within the press here. In a stinging editorial the Cape Times, which has refused to publish any of the details, accused The Star of an "extraordinary lapse" from established standards of newspaper ethics.

"In terms of established newspaper practice, extramarital affairs and such purely personal tragedies as the breakdown of a marriage remain personal and private unless and until the parties themselves resolve to go public," the Cape Times said.

The Star responded by arguing that it felt obliged, in the public interest, to disclose the official use of "dirty tricks" which it likened to Watergate. "No one is safe in a society that condones dirty tricks," it said in a front-page editorial.

Questioned about the matter in Parliament, Minister of Law and Order Louis Le Grange denied that the security police tried to smear Boesak. He said they learned of his affair with the church worker while keeping a watch on his political activities but kept the information confidential.

However, Le Grange and his police chiefs are resisting opposition calls for a judicial investigation into The Star's dirty-tricks allegations. Instead, they have lodged a complaint of unfair reporting with a press council that has no powers to summon or cross-examine witnesses.

Boesak has not been charged under the laws prohibiting interracial sex, nor is he likely to be. There has been no prosecution under the Immorality Act, which forbids sex between whites and nonwhites, for four years. Even when an affair has been acknowledged, a successful prosecution would require evidence admissible in court of an interracial sex act.

Meanwhile, Boesak has spoken of the "difficult time" he has been through and of the "utterly sick nature of society in South Africa" that the affair has revealed.

But he has said that he will neither leave the country nor slip into obscurity. "If people think I will crawl into a hole and not be seen again, they have another think coming," he said defiantly at the meeting Saturday.