All but one of the 43 mercenaries who fled the Seychelles Islands in a hijacked Air India jet after a bungled coup last November were found guilty here today under South Africa's antihijacking law.

After a five-month trial, Provincial Supreme Court Judge Neville James absolved the South African government and Prime Minister Pieter W. Botha of complicity in the coup attempt, as alleged by the mercenaries' leader, Col. Mike Hoare.

But the judge accepted Hoare's testimony that he had met with South African intelligence officers who supported the operation and that this had led to the South African Defense Force supplying Hoare with Russian-made weapons for it.

Judge James also accepted the fact that Martin Dolinchek, one of eight other mercenaries left behind in the Seychelles and sentenced to 20 years' imprisonment there July 6, was an officer in South Africa's National Intelligence Service.

The 42 convicted mercenaries will be sentenced Wednesday and could get prison terms of up to 15 years.

Hoare was found guilty on three charges, each of which carries a mandatory minimum sentence of five years. The judge may suspend part of the sentence, however.

Hoare, who became internationally known when he fought for Moise Tshombe in the Congo wars of the 1960s, presented himself throughout the trial as a soldier of honor dedicated to the cause of fighting communist intrusion in Africa.

But the judge today lambasted him as a liar, a bungler and a leader who was prepared to go back on his word to his men.

Hoare shrank visibly before the judge's tongue-lashing. During the morning tea break he was cheerfully autographing his book, "Mercenary," but by the end of the day he was slumped low in a corner of the dock, ashen-faced and scowling.

Peter Duffy, a South African press photographer who fought with Hoare in the Congo, was also found guilty on three charges.

Hoare's second in command, Italian-born Tullio Moneta, and five others were found guilty on two charges, while 36 were guilty on one charge, of jeopardizing the Air India plane and its passsengers.

The only man to get off was a 25-year-old American, Charles Dukes of Miami, a veteran of Vietnam and the Rhodesian bush war.

Dukes was acquitted of all charges even though he did most of the fighting when the group landed at Mahe Airport in the Indian Ocean island republic on Nov. 25.

Judge James said his court had no jurisdiction over what happened on the island, only over what happened aboard the plane that was hijacked and flown to South Africa.

Dukes was badly wounded in his attack on an Army barracks and was sedated with morphine before being carried onto the plane. Judge James said he could not be held responsible for any part of the hijacking.

A second American, 26-year-old Barry F. Gribbin of Miami, was found guilty on one charge, of jeopardizing the airliner's safety.

The group was recruited by Hoare in South Africa, traveled to Swaziland by bus posing as beer-drinking vacationers and caught an Air Swazi plane to the Seychelles.

Their plan was to slip through customs with automatic rifles concealed in false-bottom bags, to link up with dissidents on the main island, then to seize all the island's key points. A pro-Western former president, James Mancham, would then be flown in from Kenya to replace Socialist President Albert Rene, according to the plan.

Things went wrong when a customs officer spotted a gun in the bag of one of the last men to go through. A firefight broke out and one of the mercenaries, Johan Fritz, was killed.

In the ensuing chaos, a group of mercenaries seized the control tower. They talked down the Air India Boeing 727, which was on its way with 65 passengers from the Zimbabwean capital of Harare to Bombay via the Seychelles.

For the South African government, the judgment comes as a relief. From the outset, it faced accusations of complicity in the plot.

The suspicions began when South Africa initially failed to charge the mercenaries under its antihijacking law, charging only five with the less serious offense of kidnaping.

Only after warnings by the United States and other Western countries that they might sever air links with South Africa for violating international agreements on air piracy did the government charge all the mercenaries with hijacking.

When he testified in May, Hoare declared that the government was involved in the coup plot.

He said Dolinchek introduced him to the chief of the National Intelligence Service, Alec van Wyk, and his deputy, N.J. Claassen, and they discussed the idea. Van Wyk put it to the Cabinet, but it was turned down.

Hoare said Claassen eventually told him Prime Minister Botha had approved the coup idea in principle but ordered it handled by the military intelligence service.

Hoare said he then met two military intelligence brigadiers. They endorsed the plan, had Soviet weapons belonging to the Defense Force delivered to his house, told him to limit the number of Army reservists he recruited, and arranged for him to use a military training ground. Johannes de Beer, an officer in the commando unit from which 11 of the mercenaries were recruited, testified that he had used official military call-up forms for the men but said he had done so on his own.

Judge James, while accepting Hoare's contact with the intelligence officers, the supply of weapons by the Defense Force and Dolinchek's connections with the intelligence service, rejected all suggestions that the government itself was implicated.

He said there was no evidence to support Hoare's allegations on this and he found Hoare an unreliable witness.