LONDON — On the same day as his last public blessing Sunday, Pope Benedict XVI confronted the threat of a fresh scandal within the church hierarchy, with Vatican officials informing him of new allegations that Britain’s most senior Roman Catholic cleric had engaged in inappropriate behavior with priests.In Britain, the accusations against Cardinal Keith O’Brien — head of the church in Scotland and one of this nation’s most strident opponents of gay rights — were escalating into a national furor. The controversy revolved around a report first published Saturday night on the Web site of Britain’s Observer newspaper, saying that four men — three priests and one former priest — had denounced O’Brien this month, accusing him of engaging in “inappropriate” and “intimate” behavior.

Through a spokesman, O’Brien denied the allegations and said he was seeking legal counsel.

If proven true, the accusations could rock the church at a highly sensitive time, highlighting a Vatican in crisis as its cardinals begin to gather in Rome to pick the pope’s successor after his surprise resignation this month.

The exact nature and timing of the alleged contact, which the Observer said was reported to the Vatican’s emissary in London a week before Benedict’s Feb. 11 resignation, were not spelled out. But one of the alleged victims said O’Brien had started a “relationship” with him in the 1980s that resulted in the need for long-term counseling. Another of the men said O’Brien had initiated “inappropriate contact” during nightly prayers, according to the paper.

Poised to join the coming conclave to elect a new pope, O’Brien missed Sunday Mass in his dioceses of St. Andrews and Edinburgh. His auxiliary, Bishop Stephen Robson, read a statement at the cathedral in Edinburgh, saying: “A number of allegations of inappropriate behavior have been made against the cardinal. The cardinal has sought legal advice and it would be inappropriate to comment at this time. There will be further statements in due course.” He added, according to the BBC, that it “is to the Lord that we turn to now in times of need.”

The Vatican declined to confirm details of the allegations against 74-year-old O’Brien, who was due to retire next month, saying only that Benedict had been informed of the “problem” on Sunday and the matter was now “in the hands” of the outgoing pope.

The allegations surfaced a week after the church became the focus of fresh leaks in the Italian news media, which cited an internal Vatican report as detailing the existence of a gay lobby inside the institution that was subject to outside blackmail. Responding to the reports, the Vatican’s Secretariat of State chided the media for what it called the “widespread distribution of often unverified, unverifiable or completely false news stories that cause serious damage to persons and institutions.”

In Britain, however, the Observer report was considered additionally explosive because of O’Brien’s public stance on homosexuality. Last year, O’Brien decried the campaign to legalize same-sex marriage here as a “grotesque subversion of a universally accepted human right.” He has described homosexuality as immoral and was singled out by the London-based gay advocacy group Stonewall for a 2012 “bigot of the year” award.

Allies of O’Brien, however, were quick to defend him, saying judgment should be reserved until a full airing of the facts emerged. “These allegations have not been proved in any way,” Cardinal Cormac Murphy-O’Connor, former archbishop of Westminster, told the BBC. “So I think he will have to decide whether he goes or not” to the conclave. “We must listen first of all to what he has to say.”

The Observer’s report did not specify the extent to which the alleged encounters were consensual or ultimately resulted in sexual acts. None of the four men were named, and they could not be independently reached.

The Observer quoted one man who said he was a 20-year-old seminarian in the 1980s when first approached by O’Brien. After O’Brien was promoted to bishop, the unidentified man said, he left the priesthood “to preserve my integrity.”

“You have to understand the relationship between a bishop and a priest,” the paper quoted the man as saying. “At your ordination, you take a vow to be obedient to him. He’s more than your boss, more than the CEO of your company. He has immense power over you. . . . He can move you, freeze you out, bring you into the fold. . . . He controls every aspect of your life.”

The paper said the men recently reported their allegations to the Vatican emissary in London with the aid of an intermediary from their diocese in the week before the pope’s resignation. The move appeared pegged to O’Brien’s planned retirement next month. The men were demanding O’Brien’s immediate resignation and apparently went public in an effort to block the cardinal from taking part of the papal conclave.

The allegations surfaced at a time when the papal conclave is being cast in an unwelcome spotlight. Lay groups are orchestrating a campaign to prevent Cardinal Roger Mahony, the former archbishop of Los Angeles, from attending the conclave to choose the next pope. Mahony was stripped of his duties last month over allegations that he shielded pedophilic priests in the 1980s. Victims’ rights groups were quick to demand swift action from the Vatican on the allegations against O’Brien.

“It doesn’t matter that the offenses may have been years ago or that the victims may have technically been adults,” said Barbara Blaine, a spokeswoman for the U.S.-based Survivors Network of Those Abused by Priests. “What matters is that, once again, a shrewd, high-ranking Catholic official misused his role and status and power to harm and selfishly take advantage of vulnerable teens and young men.”

Yet church-watchers said the full impact of the allegations against O’Brien were still emerging. They said it was unlikely that the Vatican would seek to block him from attending the conclave — an event that O’Brien has previously said he would attend.

In comments made to the BBC on Friday, O’Brien said it was time to think seriously about having a pope from outside Europe and that he would be “open to a pope from anywhere if I thought it was the right man, whether from Europe or Asia or Africa or wherever.”