Cardinal Keith O'Brien 'very upset' about his resignation
The BBC has been told Cardinal Keith O'Brien is "very upset" over the circumstances of his resignation.
A source within the Catholic Church also said the cardinal "doesn't know who his accusers are and doesn't know what they're accusing him of".
The ex-Archbishop of St Andrews and Edinburgh has been accused of inappropriate behaviour towards priests in the 1980s - allegations he contests.
Cardinal O'Brien will no longer take part in the election of the new Pope.
The source also said Cardinal O'Brien, Britain's most senior Roman Catholic cleric, was "a vulnerable adult" approaching the age of 75, the age at which he was due to retire from his position.'Saddened'
At a news conference on Tuesday, Cardinal Cormac Murphy-O'Connor, a former archbishop of Westminster, said he was "saddened" by the resignation of Cardinal O'Brien.
He added Cardinal O'Brien's decision to step down was "up to his own conscience".
Cardinal Murphy-O'Connor also said the Church needed to undergo reform, and was asked whether his fellow cardinal had been right to stand down.
"It was up to his own conscience that he stepped down. He wasn't asked to, he decided to do that," he said.
"I think he thought it would be a distraction for him to be in there (at the election of the new Pope) and I think that is the main reason."
Cardinal Murphy-O'Connor described Cardinal O'Brien as a "very honest man" and said the allegations would be investigated.
He added that reform of the Church would be high on the agenda when the cardinals gathered in Rome for the conclave to elect the Pope.
"The cardinals will be discussing not just one particular issue but some of the scandals that have afflicted the Church - the gravest one has been child abuse," he said.
"There's no doubt in my mind that there has to be reform, otherwise not just the image of the church but the effectiveness of the Church is also affected."
One of Scotland's leading historians said the current crisis is the biggest the Catholic Church in Scotland has faced in about 500 years.
End Quote Prof Tom Devine
This is probably the gravest single public crisis to hit the Catholic Church in Scotland since the Reformation”
Prof Tom Devine, writing in the Daily Telegraph, said: "This is probably the gravest single public crisis to hit the Catholic Church in Scotland since the Reformation."
In his commentary piece he added: "Its effects in the short term are incalculable."
Cardinal O'Brien announced his resignation on Monday, following allegations in the Observer newspaper on Sunday.
It said three priests and one former priest, from the diocese of St Andrews and Edinburgh, had complained to the Pope's representative to Britain, nuncio Antonio Mennini, in the week before 11 February.
The cardinal's resignation statement said the Pope would appoint someone to govern the archdiocese in his place, until his successor was appointed.
Pope Benedict, 85, announced earlier this month that he is to resign, the first pontiff in more than 600 years to do so. He will formally quit on Thursday as leader of the global Catholic Church.
Cardinal O'Brien's decision to pull out of the conclave means Britain will be unrepresented. Cardinal Murphy-O'Connor, at 80 years old, is ineligible to cast a vote but he is heading to Rome to take part in pre-meetings before the conclave begins.