A former Roman Catholic archbishop and cardinal has been dismissed from the priesthood after the Vatican found him guilty of sexually abusing minors in a sign of an increasingly hardline stance taken by the church.

Theodore McCarrick, 88, one of the prominent figures in the church, was defrocked just days before an unprecedented global summit of bishops to discuss child sexual abuse is convened by the Vatican.

The Vatican’s move makes McCarrick the most senior figure to be removed from the priesthood in modern times. He will no longer be permitted to act as a cleric, and is forbidden to celebrate the sacraments except to grant absolution for sins to a person close to death. Only excommunication is a more severe punishment.

Pope Francis has approved the action as “definitive”, meaning McCarrick will not be allowed to appeal, the Vatican said on Saturday.

An earlier Vatican hearing had found him guilty of soliciting for sex while hearing confession “with the aggravating factor of the abuse of power” and ordered his “dismissal from the clerical state”.

McCarrick, who retired as archbishop of Washington DC in 2006, was forced to resign as a cardinal last July after a man publicly alleged that he had been sexually abused by the cleric from 1971, when he was a 16-year-old altar boy in New York.

McCarrick denied the claims, but the archdiocese of New York found the allegation “credible and substantiated” and turned the case over to the Vatican for investigation.

Another man subsequently claimed he had also been abused as a child by McCarrick, and several former trainee priests alleged they had been sexually harassed by the former cardinal at his New Jersey beach house.

Since September, McCarrick has been living in a friary in rural Kansas after being ordered to a “life of prayer and penance”.

Pope Francis became personally embroiled in the McCarrick case after a retired Vatican diplomat accused the pontiff of being aware of rumours about McCarrick’s behaviour but failing to take action.

In an 11-page testimony, archbishop Carlo Maria Viganò, 77, a former Vatican ambassador to the US, said there was a “conspiracy of silence not so dissimilar from the one that prevails in the mafia”.

He called on the pope to step down, saying: “Corruption has reached the very top of the church’s hierarchy.” The accusations were seized on by Francis’s conservative enemies, plunging the pope into the worst crisis of his papacy.

Next week, more than 100 bishops from around the world will meet in Rome for a four-day summit convened by the pope to address issues of sexual abuse and its cover-up.

It comes after a disastrous year for the church, with abuse and cover-up scandals emerging in multiple countries, and the pope being accused of failing to grasp the seriousness and scale of the problem.

Francis has sought to get on top of the issue. In December, he vowed that the church would “spare no effort” to bring perpetrators to justice.