The Vatican has excommunicated a rebel group of ultra-conservative Catholics who defied Pope Leo by ordaining bishops without his consent, creating a schism in the Roman Catholic church.

In a statement on Thursday, cardinal Víctor Manuel Fernández, who heads up the Holy See’s Dicastery for the Doctrine of the Faith, said the group from the Society of Saint Pius X (SSPX), founded in the Swiss village of Ecône in 1970, had “committed an act of a schismatic nature” which, under canon law, is punishable with automatic excommunication.

However, the Vatican went further than expected and said that all priests of the SSPX and all Catholics who “adhere formally” to the group were now in schism and excommunicated.

A schism is a term to indicate a severe, formal rupture within the church.

Pope Leo had made a last-ditch effort to persuade the society to halt the ordinations, which took place during a ritual-filled ceremony on Wednesday, calling them a “schismatic act” and a “sin of extreme gravity”.

But the society said the ordination of bishops who “are entirely faithful” to the Catholic church’s “tradition” were “a sacred duty”.

An estimated 16,500 people flocked to Ecône for the ceremony, including members of Forza Nuova, an Italian neofascist political party, and Futuro Nazionale, a new far-right force threatening the Italian prime minister Giorgia Meloni’s chances of winning a second mandate in general elections next year.

Despite being a splinter group, the SSPX has a wide reach, gaining a significant following in the US where it has a large operations base in Kansas, as well as in France, Argentina and other countries. The order has nearly 1,500 priests, seminarians and other vocational members.

The society rejects ​central changes that emerged from the Second Vatican Council – a landmark Vatican gathering of cardinals, patriarchs, bishops, theological experts and others between 1962 and 1965 – including allowing mass to be celebrated in local languages. Until then it had been said only in Latin.

The ordinations mark the first significant crisis for Leo because of the schism – an intentional rupture of the church’s unity. Since he was elected in May last year, the first North American pope, he has made church unity a priority and has worked especially hard to heal rifts with traditionalists, which had deepened during the papacy of his predecessor, Francis.