South Africa's raids today into neighboring black countries have probably wrecked a major peace initiative by a special Commonwealth mission and brought closer the prospect of international sanctions against this country, according to western diplomats here.

The initiative, regarded as important by Washington, was nearing the end of its most delicate week of shuttle diplomacy aimed at opening negotiations between the Pretoria government and the exiled African National Congress when the raiders struck at ANC facilities in Zambia, Zimbabwe and Botswana.

A seven-member team of Commonwealth statesmen, which met with the ANC leaders in Zambia Saturday, was about to go into a meeting with a special constitutional committee of South Africa's six most powerful Cabinet ministers this morning when the news was announced.

"The timing was abominable. It appears to spell the end of the whole initiative, and therefore of the last chance for South Africa as far as the rest of the world is concerned," a senior Commonwealth diplomat who has been closely connected with the mission remarked here tonight.

By that, the diplomat indicated a belief that the mission will now recommend that the Commonwealth, which includes Britain and most of its former colonies, apply mandatory sanctions against South Africa when it submits its report next month, and that this will set in motion a snowball of international action aimed at forcing Pretoria to negotiate an end to white minority rule.

This sentiment was echoed by the Commonwealth secretary general, Shridath Ramphal, who said in London tonight that western governments now "surely have no choice but to take serious sanctions action" against South Africa.

The mission, made up of "eminent persons" from seven Commonwealth countries, was appointed as a compromise at a Commonwealth summit in Nassau last October when all members except Britain favored sanctions.

It was told to pursue a peace initiative in the strife-torn country, and report on its prospects by the end of June before the Commonwealth reached a final decision on sanctions.

Sources close to the Eminent Persons' Group, as the mission has become known, say it is likely to present its report in advance of a U.N.-mandated world conference on sanctions against South Africa that is to be held in Paris June 16-20. This would give greater impact to any call for sanctions it may make.

The timing would also be significant in South Africa. June 16 is the 10th anniversary of the 1976 Soweto riots and has been chosen by black activist groups as the date for starting a three-day work stoppage that is expected to bring nearly two years of antiapartheid campaigning to a climax.

Until the past few days, the group's initiative looked promising.

The group had made two visits to South Africa and, by keeping a low profile, appeared to have raised its standing in the eyes of a skeptical Pretoria government, which allowed it access to the imprisoned ANC leader, Nelson Mandela.

A month ago the group put forward a peace plan aimed at opening the way for the first direct negotiations between Pretoria and the outlawed black resistance movement.

The plan, as disclosed by Ramphal, involved Pretoria legalizing the ANC and releasing its imprisoned leaders, including Mandela, in return for the ANC declaring a truce in its guerrilla struggle and pledging its efforts to end the violence that has engulfed South Africa's black townships for most of the past two years. The two sides would then sit down to negotiate a new constitutional future for the country.

South Africa sent a special envoy, Carl von Hirschberg, to London to tell the group it was not opposed in principle to releasing Mandela and legalizing the ANC -- but that if it did so the Commonwealth should condone whatever action it had to take to contain any public disturbances that followed.

At the same time sources close to the group disclosed that Mandela had also given an encouraging response to the plan, saying he regarded it as a "reasonable starting point," although he stressed this was a personal view.

These responses caused a flurry of hope in diplomatic circles. President Kenneth Kaunda of Zambia in particular threw his weight behind the plan, describing it as "the last ray of hope" for South Africa and urging the ANC -- which has its headquarters in his country -- to respond positively.

But the hopes began dimming as the Commonwealth team, headed by Malcolm Fraser, a former Australian prime minister, and Gen. Olusegun Obasanjo, Nigeria's former head of state, returned to South Africa May 11 and began shuttling between Cape Town and Lusaka, Zambia.

It held another meeting with Mandela, which sources said was again encouraging, but on Thursday, South African President Pieter W. Botha -- who did not meet with the group on this visit -- delivered a speech in which he condemned the "unsolicited interference" of "meddling groups visiting the country."

Although Botha stressed that some visiting groups were "well-meaning," which Commonwealth diplomats hoped was intended to make an exception of their team, the timing of his statement seemed pointed.

Botha made it clear that he was pressing ahead with his own agenda of cautious reforms, which most blacks reject, and he warned those opposing him that they would be confronted by the full might of the state.

He announced that he was going ahead with a plan, announced in February, to create a body to negotiate a new constitution for South Africa on which selected blacks who renounce violence will be invited to serve.

Local newspapers published leaked details of the new body, called the National Statutory Council. These indicated that it would have 20 members, including 10 blacks drawn from nonindependent tribal "homelands" and from urban black communities, all nominated by Botha as the council's chairman.

To seasoned observers, it seemed clear that such a negotiating forum would be unacceptable to the ANC.

Botha announced nonnegotiable principles for the new constitution, including preservation of group self-determination and protection of minority rights -- code phrases that are expected to invite rejection by the ANC, which opposes the concept of racially defined "group" politics.

The ANC's exiled leadership has misgivings about the peace plan and stalled on giving a direct answer when the Commonwealth group met with members of its national executive, including President Oliver Tambo, on Saturday. Congress sources said in telephone interviews that the ANC asked the group to return in about 10 days for further discussions.

To reject the proposal could make the black movement appear unreasonable, costing it support abroad and among the liberal whites it is wooing with some success inside South Africa.

But, ANC sources said, to accept it without a commitment by Pretoria to move toward black majority rule would mean ending the insurrection in the townships as it is beginning to put pressure on the white establishment, in return for being locked into a negotiating filibuster. Then, the ANC fears, it would rapidly lose the support it now appears to have in the townships.