ROME (AFP) - Pope Francis' hopes of easing the Catholic Church's tough stance on remarried couples has hit a stumbling block, with five cardinals coming out firmly against reform ahead of a key meeting, the Italian media reported on Wednesday.

The Argentine Pontiff, whose defence of the poor and vulnerable has stirred up the winds of change within the centuries-old institution, rocked traditionalists last weekend by marrying couples in Saint Peter's Basilica who had been living in "sin", according to the Church's rule book. Of the 20 couples gathered in the heart of the Vatican to take their vows with the leader of the Roman Catholic church's blessing, some had already been cohabiting and one bride was a single mother.

The debate began in earnest in February after Pope Francis' close ally German Cardinal Walter Kasper, tapped by the 77-year-old Pope to address the world's cardinals on hot-button issues, sparked controversy by calling for a more "merciful" approach to remarried Catholics who want to take communion. In a mass in his private chapel, Pope Francis also said the Church must "accompany... not condemn" those whose marriages break down.

Out of the uproar among conservatives has come a book, Remaining In The Truth Of Christ, which brings together the opinions of five leading Catholics - including Cardinal Kasper's most vocal critic Gerhard Ludwig Muller, prefect of the doctrinal watchdog, the Congregation for the Doctrine of the Faith - and four other experts. The in-house revolt comes ahead of a two-week meeting of bishops on the subject of the family in October.

The book's editor, Robert Dodaro, sums up the essays in the introduction, saying the cardinals "are united in firmly maintaining that the New Testament shows us Christ forbidding without ambiguity divorce and remarriage".

"The 'merciful' answer to divorce endorsed by Cardinal Kasper is not unknown in the old Church but none of the authors here, which we consider to be authoritative, defend it. On the contrary, when they mention it, it is rather to condemn it as going against the scriptures," he writes.

"While there can be occasional transgressions, the fact they exist does not mean they are not transgressions, or that they are models to be followed," he said.