Poland's spiritual leader, Cardinal Stefan Wyszynski, is suffering a prolonged illness, giving rise to deepening concern in the country about his eventual recovery.

Attention to the health of the primate, who turns 80 in August, is especially keen as a result of the Roman Catholic Church's critically important role in keeping the peace here. Wyszynski, who has been primate for 31 years, has personally served as a key mediator between the workers' movement and communist authorities during the past nine months of social upheaval.

The cardinal is said by church officials to have fallen ill a week before Easter due to a digestive problem. The illness, which forced him to miss both Easter services and last Sunday's emotional celebration of Poland's 1791 democratic constitution, has developed complications as a result of what church spokesman Alotzy Orszulik termed a "cold."

Another church official privately told Solidarity union leader Lech Walesa and a group of others gathered last weekend at Poland's holiest shrine -- the Jasna Gora monastery in Czestochowa -- that a medical test on the primate for cancer of the colon had proved negative but that cancer of the pancreas was among the sicknesses that cardinal may be suffering.

The church has issued only a few brief official statements on the cardinal's health, explaining simply that he is ill and being attended by physicians. During church services on the past two Sundays, standard prayers for the health of the primate were said for the first time with the primate's name included -- a fact cited as significant by some observers.

Orszulik in a brief interview sought to dampen concern. "Usually he's been sick every spring," the spokesman said of the cardinal. But he added, "It is obvious that when you are reaching the age of 80, anything can happen." Orszulik also said the primate has been seeing only his closest associates.

Speculation about a possible successor to Wyszynski has gone on for months. Among those most often mentioned are: Cardinal Wladyslaw Rubin, prefect of the Congregation of Eastern Churches in the Vatican; Archibishop Franciszek Macharski of Krakow, and the Rev. Jozef Tyschner, vice dean of the Papal Faculty of Theology in Krakow and a favorite among Catholic intellectuals.

The Warsaw government also has a strong interest in who will head Poland's powerful church. Under a 1956 decree, the government can influence the selection of Polish bishops by exercising a limited veto power over the list of final candidates presented by the church. However, Orszulik said that church-state talks were under way that, he stated, would eliminate this decree.

This is just one indication of the gain in prestige and power enjoyed by the Catholic Church as a result of the politically astute course it has steered between the old and new institutions of power.

On the one hand, church officials have assuaged communist worries by often stressing the necessity for law and order. The government, in turn, has appeared more willing to give way on -- or at least discuss -- a number of longstanding church demands including access to the media and religious ministry in state-run hospitals and prisons.

On the other hand, the church has kept the confidence of workers by backing their demands for freedom. This has served to reaffirm the strong identification seen between Catholicism and nationalism in a country where 80 to 90 percent of the 36 million people are Catholic.