In a development that could weaken the ability of African guerrilla leaders to enforce a cease-fire in Rhodesia, Gen. Josiah Tongogara, the overall commander of the Patriotic Front military forces, was killed in an automobile accident in Mozambique, it was announced today.

Patriotic Front coleader Robert Mugabe, whose forces Tongogara commanded, said today in Mozambique that he believes the general's death was an accident, according to radio reports monitored here.

The problem is that most of Tongogfara's 12,000 to 15,000 guerrillas inside Rhodesia, faced with the decision of laying down their arms tomorrow, are likely to believe that their leader was a victim of foul play.

Announcement of Tongogara's death, which leaves the Patriotic Front military command leaderless, was the most serious development on a day in which there were growing indications that Britain would have considerable difficulty in implementing the cease-fire. The truce was worked out in London as part of an overall settlement to bring about a black-majority government.

Three British soldiers, members of the 1,300-man Commonwealth cease-fire monitoring force, were killed when their helicopters hit power lines and crashed 100 miles northwest of Salisbury. British military officials said there was no indication that the crash was caused by hostile fire.

However, a C130 transport, deploying monitoring forces and Patriotic Front commanders to areas where the guerrillas are supposed to assemble, was hit the guerrilla rifle fire east of Salisbury, according to the British pilot of the plane. There were no casualties.

Details about Tongogara's death were sketchy and conflicting, a fact that is bound to lead to further suspicion that it was not an accident Mugabe's Zimbabwe African National Union announced in Maputo, Mozambique, that Tongogara, 41, had died instantly and three other military leaders were injured when their vehicle ran into a bus in central Mozambique.

No information was given on when the accident occurred or the exact location except that Tongogara had been heading for his military head-quarters in Chimoio to brief commanders on the British-sponsored cease-fire that is supposed to take effect in Rhodesia at midnight tomorrow.

The date of the accident was variously reported as Christmas Day, yesterday or today.

Voicing concern over the impact of the incident on peace prospects, diplomats here pointed out that past violent deaths of guerrilla leaders make it likely that Tongogara's death will be regarded in the same light, even if it was an accident.

In the suspicious environment that exists in Rhodesia, "one cannot suspect that anything is accidental," one diplomat said.

Tongogara's death could have a significant impact on prospects for a cease-fire and election of a black majority government early next year whether it was accidental or caused by either internal division within the Patriotic Front or by the white-backed Salisbury fraction of Bishop Abel Muzorewa.

Under the crease-fire plan adopted last week by the warring sides, all forces are to be grouped at assembly points or bases by Jan. 4, one week after the cease-fire date.

ZANU has reportedly already told the newly installed British colonial administration in Salisbury the Tongogara's death will delay that time-table. a

He had been scheduled to arrive in Salisbury tomorrow to help the Commonwealth cease-fire monitoring force assemble his troops at the designated point. Having borne the brunt of the seven-year war, his ZANU forces are regarded as more militant and less likely to assembly than those in the other wing of the Patriotic Front, Joshua Nkomo's Zimbabwe African People's Union.

Tongogara, virtually unknown in the West emerged at the London talks as a major moderating force faboring a peaceful settlement. British hopes for a successful cease-fire in the guerrilla war partly pinned on Tongogara's ability to persuade his ZANU forces to join in the peace process.

Tongogara also played a key political role in ZANU and was a leading force urging unity with ZAPU. The removal of Tongogara increases the chances for division within the Patriotic Front.

Mugabe and Nkomo have been expected to meet this weekend in Dar es Salaam, Tanzania, to work out whether they will run as a coalition or separate parties in elections scheduled to be held in late February or early March.

Their decision, which now may be delayed, could have a major bearing on the outcome of the election, since a divisive fight could boost Muzorewa's chances. Mugabe has talked of running separately despite calls for unity from Nkomo and African supporters of the Front.

A ZAPU official based in Lusaka reflected this concern, saying, "We are stunned. This is a serious setback."

Some analysts felt that Tongogara was the real power in the complicated political structure of ZANU with Mugabe being somewhat of a figurehead.

The bearded Tongogara, a giant of a man, towered over the diminutive Mugabe, who was invariably at his side at frequent Patriotic Front press conferences during the 15-week London talks. On the other side was the rotund Nkomo, making Mugabe look even smaller.

At one such press conference last month, Tongogara reached over and calmed Mugabe during a particularly heated outburst when the ZANU leader said British Foreign Secretary Lord Carrington "could go to hell" over an ultimatum on British cease-fire proposals.

Tongogara was born in 1938 at Selukwe in western Rhodesia and as a boy worked on a farm owned by the parent of Ian Smith later the prime minister of Rhodesia who led the country into illegal independence and protracted war against Tongogara's forces.

When he met Smith at the London conference he inquired about his mother's health and was impressed to learn she was 86 and in fine fettle.

He reminisced with Smith about how Ms. Smith used to give candy to the African children working on the farm.

"If I get home while the old lady is still alive," he told an interviewer, "that would be one of the greatest things for me -- to say hello, ask her about the sweets and whether she still has got some more for me."