Juan D. Peron's widow and presidential successor, Isabel Peron, has "irrevocably" resigned as head of Argentina's divided Peronist party, its leaders here said today.

From voluntary exile in Spain, Peron wrote letters to the heads of the party's two factions announcing her decision. The local leadership confirmed the authenticity of the letters -- dated Feb. 4, her 54th birthday.

Peron's chaotic presidency, not quite two years in length, ended in a military coup in 1976. When the military gave way to elections in late 1983, the Peronists, under Peron's nominal party leadership, lost their first election to the Radical Party of current President Raul Alfonsin.

Observers said Peron's move meant the disappearance from the political stage of a name that had dominated Argentine public life for the nearly four decades. Juan Peron had no children by any of his three wives.

The decision came in the aftermath of a historic split early this month in the once monolithic, if disparate, movement. Since Juan Peron's death in 1974, analysts here say, no one has emerged who could keep together a membership ranging from Marxist left to neofascist right.

The resignation also leaves the rule of Alfonsin without one of its few vocal supporters among the Peronists. Government officials have watched with a curious mixture of glee and gloom as Peronists struggled to find a leader.

Peronist leaders already have expressed concern about the prospsects of the Justicialist Party -- the formal name coined by Juan Peron in his first presidency 40 years ago -- in congressional elections later this year.

In November, the party suffered a second electoral reverse at the hands of Alfonsin as voters ignored its counsel to abstain in a plebiscite on a peace treaty with Chile. More than 80 percent of eligible voters turned out, and most supported the government's initiative.

Peron's one- paragraph, handwritten letters also were dated two days after the emergence of a formally split party at separate conventions here and in the provincial city of Rio Hondo. While most Peronists had few illusions that the widow could lead the mass movement, such was the enduring strength of the Peron name that both factions voted her into their party presidencies.

The ex-president's administrator of personal property, who delivered the letters, said the reason for the delay was a mail strike here.

"The resignation," said Santa Fe Province Gov. Jose M. Vernet, first vice president of the labor-dominated old-guard wing of the party, "is a call to attention for all Peronism." Peron's presence, he said, "had meant the possibility of unity."

For die-hard Isabel Peron loyalist Juan Labake, however, Peron's message had a different meaning. "She's quitting both factional presidency positions as a way of showing her disapproval of both groups, which are trying to use her name for their own ends," Labake said in a telephone interview.

However, Labake, known for his elaborate theories about sphinxlike political behavior, emphatically denied that the resignation meant the end of her political career.

"This is just a maneuver to produce unity by means of having new leadership come forth," he said. "Mrs. Peron feels the mafia running the party now is leading it to oblivion. Peronism needs new blood."

Political observers pointed out that the resignation -- which bypassed the so-called Peronist Superior Command of party notables set up by Peron during a visit last May -- was yet another in a series of baffling gestures taken by the hermetic exile from her refuge in Madrid, where she has resided for the past four years.

Maria Estela Martinez de Peron, known by the diminutive of her stage name as "Isabelita," is often referred to as a tragicomic figure who never fully emerged from the shadow of "Evita," Peron's beautiful second wife. The idol of the "shirtless ones," Eva Peron won over Argentina's masses with a blend of demagoguery and populism.

Isabel Martinez met Peron while working the Happyland nightclub in Panama City when the former strongman was in forced exile following a coup in 1955. Although she later became the outcast populist's personal delegate on a number of political trips to Argentina, Isabel Peron remained much in the shadows until her surprise nomination for vice president on the ticket led by her ailing husband in 1973.

Upon assuming the presidency in 1974, Peron presided over a wobbly government still vividly recalled here for its 1,100 percent annual inflation, a social welfare minister who ran paramilitary death squads out of his office and Peron's many hysterical outbursts during televised addresses.

Many analysts say Peron would have been relegated to oblivion had she not been arrested and jailed for 5 1/2 years by the military that overthrew her. Her imprisonment, on corruption charges, became a rallying point for her sharply repressed party, and her exile to Spain in July 1981 was considered a psychological victory for antigovernment forces.

Her public support for the Alfonsin government -- her first visit to Argentina from her Spanish exile was to attend his inauguration -- had also contributed to a political rehabilitation. During that trip, and a second one last May, Peron repeatedly told party leaders their first duty was to "support democracy."