Two of South Africa's premier black newspapers, the Post and Sunday Post, have been forced to close permanently because of a government decision to ban them if they resume publication, the giant Argus Company, which owns them, announced today.

Both newspapers were closed temporarily late last year when their registration lapsed during a two-month strike by black media workers. The strike prevented the newspapers from fulfilling a requirement under South African law to publish at least once a month.

Following settlement of the strike, Argus applied for reregistration of its newspapers. Until today it was assumed that the government would not block their publication, although in retrospect its banning of three black journalists on the day that the application was submitted was a clear warning of what was to come. In South Africa, banning is a form of political restriction that often includes house arrest.

A joint statement released today by the minister of internal affairs, Chris Heunis, and the minister of justice, Kobie Coetzee, made clear their intention to ban the Post and Sunday Post under the Internal Security Act if the Argus Company persisted with its application for registration.

In view of the government's decision, Argus decided to withdraw its application for registration of the Post and Sunday Post. Hal Miller, managing director of the company, said of its decision: "Another bar has been added to the cage which is beginning to circumscribe our freedom."

The decision drew widespread condemnation, with criticism from journalists who support the black-consciousness movement and pro-government Afrikaans-language editors.

The Media Workers' Association, which represents nearly all South Africa's black journalists, described it as "an attempt to stifle black opinion" and "a declaration of war against black aspirations." An Afrikaner editor, Ton Vosloo, condemned it as prempting the findings of the government-appointed Steyn commission of inquiry into the media.

In a front-page editorial, The Star, South Africa's largest circulation newspaper and the Argus Company's main newspaper, summed up its feelings under the headline: "A New Act of Folly."

It said: "Freedom has been further curtailed. Bitterness among sophisticated blacks has increased. Putting Post on ice will not freeze the political turmoil in the townships, nor will it give any government more time to find solutions. Instead the state will aid extremists and discomfort many moderates."

The Post and Sunday Post came into existence in 1978 to fill the vacuum caused by the banning of The World and Weekend World in October 1977. Although owned by a white-controlled company, both sets of newspapers sought to reflect black feelings and to campaign for the redress of black grievances.

The Post's sales were running at about 112,000 a day. With more than 900,000 readers as distinct from buyers it was the most widely read newspaper in South Africa. Sunday Post had a circulation of about 118,000 and an estimated readership of 1.2 million.

Although circulation of these newspapers lagged behind that of The World (147,000) and Weekend World (204,000) when they were outlawed in 1977, Post and Sunday Post attracted able black journalists and were gradually approaching the position occupied by their predecessors.