The Vatican says it is alarmed by reports from China that Roman Catholic bishops there are to be forced to attend what it calls the illegal ordination of a new Catholic bishop.
The consecration on Saturday is being sponsored by the Beijing government.
China's millions of Catholics are split between followers of Pope Benedict XVI and members of a state-backed church.
The Vatican and China have had no diplomatic ties since the 1950s, when Beijing expelled foreign clergy.
Relations between Rome and Beijing - which had been improving in recent years - appear to have suffered a major setback.
Although official diplomatic relations between Rome and Beijing have been frozen for more than half a century - since the Communist takeover of China - the Vatican and the so-called Patriotic Chinese Church supported by the government had reached a tacit agreement on the appointment of new bishops.
Now this agreement is threatened by reports that the consecration of a new bishop is to take place in China's Hebei province without the approval of Pope Benedict.
The Vatican says it would regard a reported attempt by the Chinese authorities to force Chinese Catholic bishops, who are in communion with Rome, to attend the ceremony as a grave violation of freedom of religion and of conscience.
It would also damage relations between China and the Vatican, the Pope's spokesman Father Federico Lombardi said.
There are an estimated 10 million Catholics in China, most of whom belong to the Patriotic Chinese Catholic Church.
But there are several million more who belong to an underground Catholic Church, which accepts the sole authority of the Pope in Rome to appoint new bishops.