The sinking of a ship belonging to environmental protesters campaigning against French nuclear policies has posed a delicate political dilemma for right-wing French opposition parties who usually are eager to seize on any issue to embarrass the ruling Socialists.

The caution with which opposition politicians have reacted to charges that French secret service agents blew up a Greenpeace ship in the New Zealand port of Auckland July 10, killing a man aboard, has reflected one of the unwritten rules of political debate here: Thou shalt not bring into question the force de frappe, France's independent nuclear deterrent.

Earlier this week, President Francois Mitterrand's right-wing opponents broke a self-imposed silence on the affair, which has been making headlines here since early August. But rather than attack the government for authorizing a criminal act in a friendly country, they instead charged incompetence.

While two French secret service agents prepare to go on trial in New Zealand on charges of arson and murder, the political debate in Paris has centered on who was responsible for allowing them to get caught.

The French press today named a suspected informer aboard the sabotaged ship, the Rainbow Warrior, as Lt. Christine Huguette Cabon, an employe of the General Directorate for External Security (DGSE), the French equivalent of the CIA.

Along with several other alleged French agents, Cabon has disappeared since the sinking of the ship. She reportedly infiltrated Greenpeace under a false name last spring and provided inside information to the DGSE.

In the National Assembly, Jean-Claude Gaudin, floor leader of the center-right Union for a French Democracy party, called for the resignations of Prime Minister Laurent Fabius and Defense Minister Charles Hernu.

"Because of your clumsiness, a boatful of environmentalists have been given the chance to launch an unprecedented propaganda campaign. This is a typically Socialist action," Gaudin said.

Michel Bassi, a right-wing commentator, complained in Le Figaro: "When we were in power, we would have been more successful."

Charles Pasqua, the neo-Gaullist leader in the Senate, complained that French prestige had suffered as a result of the Greenpeace affair and the secret services felt "abandoned and humiliated."

The continued silence of other opposition figures has officially been explained as a wish not to prejudge the investigation headed by a prominent civil servant, Bernard Tricot, who is expected to present his report next week.

Most French commentators, however, say they are convinced that the opposition leaders do not want to lay themselves open to accusations of exploiting an extremely sensitive issue of national security for partisan political reasons.

A dissenting voice in the opposition ranks was provided by a centrist deputy, Bernard Stasi, who described as "totally cynical" the idea that France was right to sabotage the Rainbow Warrior but wrong to get caught. Stasi said that, while serving as minister for France's overseas territories in 1973, he had opposed an earlier plan by "certain authorities" to sink a Greenpeace ship.

When Mitterrand ordered the armed forces last weekend to prevent "by force if necessary" any future attempt by Greenpeace to disrupt French nuclear testing in the Pacific, the opposition could only sit back and applaud as he publicly pledged to continue the tests in defiance of the wishes of most of the countries in the region.

Mitterrand's statement appeared to be the first stage in a counterattack by the Socialists that continued this week with Hernu, who has political responsibility for the running of the secret services, dismissing rumors that he would resign. Hernu described reports that he might be sacrificed by Mitterrand as "extremely amusing."

The political situation could change next week if the Tricot report finds that the DGSE did organize the sinking of the Rainbow Warrior. Any attempt by the government to limit the political fallout of the scandal by pinning the blame on wayward elements in the secret services would be resisted by both the right-wing opposition and senior members of the DGSE.

Before appointing Tricot, who was chief of staff in the presidency of Charles de Gaulle, to head the inquiry, Mitterrand promised to make his report public and to dismiss any official "at whatever level" found to have ordered the operation in New Zealand.

In addition to protecting himself from attack on the right, Mitterrand must also be careful to safeguard his left flank. This week, his own Socialist Party condemned "without reserve" what it described as "the criminal operation" against the Rainbow Warrior, insisting that nothing could justify such a "terrorist action."

The Communist Party, which left the government last year, has accused Mitterrand of responsibility for the sinking of the ship and demanded greater parliamentary control over the secret services.