Cardinal Angelo Becciu, the most senior Catholic Church official ever to stand trial before a Vatican criminal court, was sentenced to five-and-a-half years in jail in December 2023. He denies all wrongdoing and is free pending an appeal.
Francis, who died on April 21 at the age of 88, had fired Becciu from a senior Vatican job in 2020 and accused him of embezzlement.
The Pope allowed Becciu to keep his ecclesiastical title and his Vatican apartment but stripped him of what the Vatican said at the time were "the rights associated with the Cardinalate", leaving ambiguity over whether he could join the conclave.
"Having at heart the good of the church ... I have decided to obey, as I always have, the will of Pope Francis and to not enter the conclave, still remaining convinced of my innocence," the cardinal said.
The resurfacing of the Becciu issue could deal a blow to Italian Cardinal Pietro Parolin, a senior Vatican official who is seen as a leading candidate to succeed Francis.
The Becciu case centred on the messy $US200 million purchase of a building in London by the Secretariat of State, the Vatican's key administrative and diplomatic department, headed by Parolin, who has not been accused of any wrongdoing.
About 135 cardinals under the age of 80, from all corners of the world, are eligible to take part in the vote for the next head of the 1.4-billion-member church, which is beset by concerns over its finances and divisions over doctrine.
The previous two conclaves, held in 2005 and 2013, lasted just two days but there have been suggestions that things might take longer this time.
Some of the cardinals appointed by Francis, who appointed many cardinals from countries who had never had them before, had not previously met.
However, Cardinal Gregorio Rosa Chavez of El Salvador said he felt this conclave could be wrapped up swiftly.
"I have the impression that the conclave will be short, two or three days, this is the feeling we have inside the room," he told reporters. Given his age of 82, Chavez will not be eligible to vote.
Cardinal Louis Raphael Sako, head of Baghdad's Chaldean Catholic Church, also said he expected a similarly short process to elect a successor to Francis, who was the first Pope from Latin America.
"There is a fraternal and sincere atmosphere, so it is the spirit of responsibility to choose someone who would continue Pope Francis' work," he said.
