South African Prime Minister Pieter Botha announced today that white voters would go to the polls in a general election on April 29, more than 18 months before an election is due. It will be the fourth successive early election since 1970.

Botha's announcement in parliament came after he had cast aside his relatively reformist image to adopt a tough right-wing stance. It was preceded by a speech in which he ruled out any possibility of blacks serving in the President's Council and also disclosed that a Soviet KGB agent had been captured in South Africa last year.

A major reason for calling a general election was that by-elections are pending to fill 17 vacant seats in parliament and 13 in the provincial councils. Most of the vacant parliamentary seats resulted from the promotion of deputies, including four Cabinet ministers, to the President's Council.

According to informed observers, Botha was fearful that if he held the by-elections first and waited until next year for the general election he would have put himself at a disadvantage in his battle against the ultraconservative Herstigte (Reconstituted) National Party.

The Herstigte Party has been exploiting white uneasiness about some the botha's reformist measures, the most recent being the decision to give hotel and restuarant owners the discretion to admit blacks. However anemic these measures seem from afar, they have aroused white fears, and the ultra-conservatives have been quick to capitalize on them.

Furthermore, Botha is said to be anxious to win a mandate from the white electors in his own right. He was elected premier in September 1978 and inherited the massive majority won by his predecessor, John Vorster, in the general election of 1977, when the ruling National Party captured 135 out of parliament's 165 seats and won more than two-thirds of the white vote.

Two more factors may have influenced the timing of the election decision: the impasse with the United Nations on Namibia and the banning of the black-edited newspapers Post and Sunday Post. South Africa's hard stand on Namibia and the banning of the newspapers have given Botha's administration the tough image he needs to deal with right-wing critics from both the Herstigte Party and within his own National Party.

Yesterday, two more leading Post journalists, both executives of the Media Workers' Union of South Africa, were "banned" by the authorities. The bannings of Phil Mtimkulu and Joe Tholoe bring to five the number of prominent black journalists banned within a month. Banning orders prohibit them from entering black townships except the one they live in, and keep them under house arrest at night and on weekends.

The leader of the opposition Progressive Federal Party, Frederick Van Zyl Slabbert, said today that Botha enjoyed one of the biggest parliamentary majorities in the world and did not really need a bigger majority to bring about reform. "Mr. Botha apparently needs an election to play for time, and he is doing it at a stage when there is very little time left to bring about changes."

Against Slabbert's view is that of observers who believe -- or hope -- that once Botha has a mandate in his own right, he will proceed apace with his reform program.