Argentina's ruling military junta has reconfirmed a decision not to allow former president Hector J. Campora, now gravely ill at his asylum in the Mexican ambassador's residence, to leave the country for medical treatment, according to military sources here.

The decision could rebound against the military government, observers here believe since Campora's death would be likely to become as issue for the junta's opponents.

For the past 3 1/2 years, since the military toppled the government of Isabel Peron, Campora has been a virtual prisoner in the Mexican ambassador's residence here, where he was granted aslyum after the March 1976 coup.

Argentina's military junta has refused since then to give Campora a safe conduct pass to the airport despite his urgent need for medical treatment. An Argentine physician who examined Campors in early September at the resquest of Mexican Ambassador Jose Antonio Lara Villarreal found that Campora 70; "almost certainly" has cancer.

Five Argentine military doctors sent by the junta have also examined Campora within the last month, although it is understood that Campora, a dentist as well as a peronist politician, refused to allow them to perform a biopsy for fear that they would poison him.

Those who have seen the former president, who served for only 49 days in 1973 before he resigned to pave the way for Juan D. Peron's return to power, say he is so weak he is barely able to leave his bed.

Campora's wife said last week that her husband is losing weight. She also said she will hold the government responsible if her husband dies adding that it is impossible to treat him adequately in the ambassador's home.

Campora's current plight stems from a general amnesty for political prisoners, including many proven terrorists, that he granted during his brief term as president. The military here consider him a criminal and blame him for the upsurge in terrorism that began soon after the political prisoners were freed six years ago.

There has been much speculation, in the press here in recent weeks that the junta was about to grant Campora safe conduct out of the country. This specalation intensified after newspaper publisher Jacabo Timerman, another man detained in Argentina by the government, was allowed to leave for Israel.

But not even the possibility that Campora will die in the Mexican ambassador's house and thus become a couvenient martyr for the Peronists, has yet persuaded the junta to overcome its hatred for the former president.

On Saturday, military sources were quoted in several Buenos Aires newspapers as saying that the junta had again decided against allowing Campora to leave Argentina. Instead, these source said, the junta reverted to an earlier offer to grant extraterritoriality to a hospital here where Campora could be treated while still technically under the protection of the Mexican government.

This suggestion has been rejected by the Mexicans, who maintain that international treaties that argentina has signed require the military government to give Campora safe conduct out of the country.

Lara Villarreal has also indicated a lack of confidence in the government's guarantee that Campora would not be kidnaped or otherwise harmed if he left Mexican property in Buenos Aires without a formal safe conduct pass directly to the airport.

Meanwhile, Campora's cause has been taken up recently by the Peronists, who threw Campora out of the party in 1975 and ignored him afterward because of his alleged lack of support for Mrs. Peron's governement.

Last week, Mrs. Campora appealed to the papal nuncio in Buenos Aires for assistance, and a spokesman for Pope John Paul II said the Vatican would look sympathetically on the request for papal help in securing Campora's release on humanitarian grounds.

Also in the Mexican ambassador's residence with Campora are his son, Pedro, and Juan Manuel Abel Medina, a former leader of the Peronist youth movement.