LONDON — The Catholic Church, one of the few trusted institutions in Congo, has determined that a leading opposition candidate won this week’s presidential elections, a senior Western official and a presidential adviser said on Friday, setting up a potential confrontation with the Congolese government.

The candidate, Martin Fayulu, a United States and French-educated oil executive, had led President Joseph Kabila’s handpicked successor, Emmanuel Ramazani Shadary, by nearly 30 points in recent polls. The Western official, who had talked to church officials, spoke on condition of anonymity, citing diplomatic protocol.

The Kabila government also confirmed that the church had identified Mr. Fayulu as the winner. Kikaya Bin Karubi, a diplomatic adviser to the president, said in an interview on Friday that the governing coalition party was “aware” that the church had “chosen to publish the winning candidate as Martin Fayulu.”

Mr. Kikaya said the church was breaking constitutional and electoral laws and was looking to start a “popular revolt that it will end up being responsible for.”