A rebel Catholic group at the heart of major controversies that plagued former Pope Benedict has begun criticising his successor Pope Francis for the popular approach he has taken since his election last month.

In a letter to supporters this week, the head of the ultra- traditionalist Society of Saint Pius X (SSPX) asked whether the new pontiff’s focus on serving people could be only “man-centred philanthropy” rather than true religious leadership.

Bishop Bernard Fellay’s sharp criticisms of the Vatican attracted attention during Pope Benedict’s papacy because the now retired head of the Roman Catholic Church wanted to reintegrate the once-excommunicated group fully into the Roman fold.

Pope Francis, the former Argentine Cardinal Jorge Bergoglio, has upset many Catholic traditionalists by eschewing Vatican pomp, presenting himself as a humble servant of the poor and showing little interest in returning to centuries-old traditions.

In his letter, Fellay urged Pope Francis “not to allow souls to perish because they no longer learn sound doctrine”, by which he meant the ultra-traditionalist views the SSPX advocates.

“What good is it to devote oneself to serving people if it hides from them what is essential?” asked Fellay, whose group claims 500 priests and a million followers around the world.

Aiding the poor has always been a concern for the Church, he said, “but if it becomes merely man-centred philanthropy, then the Church is no longer carrying out its mission”.

Fellay, who had long hoped that Pope Benedict would give in and reintegrate his group without conditions, had avoided public comment on his successor until now.

Soon after his election, the SSPX head for South America, Rev Christian Bouchacourt, denounced Pope Francis’s simple style as humiliating and undignified for the Church.

While Pope Benedict criticised some reforms of the 1962-1965 Second Vatican Council, Pope Francis praised it on Tuesday as “a beautiful work of the Holy Spirit” that remained to be fully implemented although some wanted to turn back the clock.