JOHANNESBURG, OCT. 24 -- The African National Congress today postponed until June a major national conference, raising concern among government officials about possible delays in negotiations over a new South African constitution that would allow blacks to share power with whites.

The conference, which had been scheduled for December, would mark the first such gathering inside South Africa by the ANC, the nation's most prominent black nationalist group, since the organization was banned 30 years ago. The ANC said it would instead hold a "consultative conference" in December to discuss political strategy.

Government officials have expressed concern that delays in the negotiations, which had been expected to begin early next year, may make it difficult to reach agreement on a new constitution before the next general election, in 1994.

Constitutional Affairs Minister Gerrit Viljoen has said the negotiations are likely to take two years. Under his timetable, an additional year would be needed for the government to hold a referendum, enact a constitution and prepare for a general election with a larger, multiracial electorate.

ANC spokeswoman Gill Marcus said the six-month postponement need not delay the start of constitutional negotiations. But she acknowledged that problems may arise if the ANC enters the negotiations under its present leaders and then reorganizes its leadership at the June congress, as is considered likely.

The ANC said the postponement stemmed mainly from delays in reaching an agreement with the government to allow about 20,000 members to return from exile. Marcus said the organization objects to holding the congress without these members and is not prepared to give the government a veto over who may attend.

At the national meeting, Nelson Mandela, 72, now ANC deputy president, is considered likely to be named president to replace Oliver Tambo, who has been in poor health following a stroke. A key decision facing the congress would then be choosing a successor to Mandela as deputy president, a post regarded as that of heir apparent.

In another development, the President's Council, an advisory panel whose majority is appointed by the ruling National Party, recommended that the new constitution provide for a bicameral parliament, with a lower house elected on the principle of one person, one vote and with a Senate representing racial groups. It proposed that the two houses have equal legislative status, with veto powers vested in the Senate.

Although the council's proposals have no official standing, they are thought to reflect government thinking. The recommendations appeared counter to impressions given recently by some officials suggesting that government leaders had dropped demands for special constitutional protections for ethnic groups. Most black political organizations oppose such "group rights," arguing that these would lead to special privileges for whites.

Meanwhile, the Pan-Africanist Congress, a major black group, appeared to face a leadership crisis with the death Tuesday night of its president, Zephania Mothopeng.