The cardinals from Asia, Europe, Africa, the United States and Latin America said on Monday they had sent the pope a set of formal questions, known as "dubia" ("doubts" in Latin), about the gathering.

In an open letter to Catholics, they said they had announced their challenges "so that you may not be subject to confusion, error, and discouragement but rather may pray for the universal Church".

The action was the latest latest clash between the pope and minority conservatives, who have accused him of undermining a number of traditional precepts.

Church leaders have been preparing for this week's closed-door meeting - known as a bishops synod - for the past two years, asking Catholics around the world to share their vision for the future of the Church.

Topics will include the role of women, greater acceptance of LGBT Catholics, social justice and the effects of climate change on the poor.

At the meeting, around 365 "members", including cardinals, bishops, lay people and, for the first time, women will vote on proposals.

Discussions will run through this month and resume in October 2024. A papal document will follow, most likely in 2025, meaning changes in Church teaching, if any, would be a long way off.

The five cardinals - all well-known critics of the pope who range in age from 75 to 90 and no longer hold any major office - are Raymond Burke of the United States, Walter Brandmueller of Germany, Joseph Zen of Hong Kong, Robert Sarah of Guinea and Juan Sandoval Iniquez of Mexico.

The are 242 cardinals in the Church and it was not clear if the five had asked any others to join. One well-known critic of Francis, German Cardinal Gerhard Mueller, was not among the signatories.

'CONFUSION, ERROR AND DIVISION'

Burke told the National Catholic Register, a conservative publication, the cardinals had sent the pope an earlier set of questions and he had responded a day after receiving them in July.

Not satisfied, they reformulated and resubmitted them to elicit a simple yes or no answer. The Register said the pope has not yet responded.

The Vatican later published the pope's seven pages of responses to the first set of questions in July. One Vatican source, speaking on condition of anonymity, said it was to show that the pope had taken them seriously and the source criticised the cardinals for behaving "as if this was a game show" of 'yes' or 'no' responses.

One question asked if same-sex couples could receive blessings, which some priests have done in a number of countries, notably Germany, even though the Vatican ruled against such blessings in 2021.

The cardinals said they wanted a clear reaffirmation of the Church's teaching that homosexual acts are sinful.

They also sought more clarity on the Church's ban on women priests, even though Francis, citing a 1994 ruling by Saint Pope John Paul II, has already said the "door is closed" on that issue.

Proponents of the synod, including many cardinals and bishops, have welcomed the consultations as an opportunity to change the Church's power dynamics and give a greater voice to lay Catholics, including women, the LGBT community and people on the margins of society.

Conservatives have taken issue with the fact that many lay people will have voting rights in a what is formally a Synod of Bishops. They say the Church should remain a male-dominated hierarchal structure where authority, particularly on doctrine, should flow from the top.

Last week, the conservative U.S.-based Catholic women's group Restore Tradition decried the fact that women were given a vote at the meeting equal to that of a cardinal or bishop, saying some of the women had advocated "heretical doctrines" against tradition.

"We wish to be represented only by bishops," the group said, citing "divine law" that only men could receive sacramental ordination.

The international conservative group, "Tradition, Family and Property," sent participants a 100-page booklet calling the gathering a dangerous "Pandora's Box". In the forward, Burke says he fears the synod will sow "confusion and error and division".