VATICAN CITY — Cardinal Keith O’Brien, Britain’s highest-ranking Catholic leader, recused himself on Monday from taking part in the conclave to elect the next pope after being accused of improper conduct with priests — an unprecedented first head to roll in the mudslinging that has followed Pope Benedict XVI’s decision to resign.

O’Brien also resigned as archbishop of St. Andrews and Edinburgh, though the Vatican insisted that Benedict accepted his resignation purely because he was nearing the retirement age of 75 — not because of the accusations. But O’Brien himself issued a statement Monday saying he would skip the conclave because he didn’t want to become the focus of media attention at such a delicate time for the Catholic Church.

O’Brien has said through his spokesman that he is contesting allegations made Sunday in a British newspaper that three priests and a former priest had filed complaints to the Vatican alleging that the cardinal acted inappropriately with them. The Observer newspaper did not name the priests, but it said their allegations date back to the 1980s. There were no details about the alleged inappropriate behavior.

It is the first time a cardinal has recused himself from a conclave because of personal scandal. It comes in the wake of a grass-roots campaign to shame another cardinal, retired Los Angeles Cardinal Roger Mahony, into refraining from participating because of his role protecting sexually abusive priests.

Mahony, however, has defiantly said he would participate in the voting for the new pope.

The difference in cases boils down to the fact that O’Brien himself was accused of improper behavior, whereas Mahony has been shown to have covered up for other priests who raped and molested children — a distinction that has long shielded bishops accused of cover-up from Vatican sanction.

Several other cardinals who will elect the next pope have been accused — and some have even admitted — to having failed to protect children from abusive priests; if all of them were to recuse themselves for negligence, the College of Cardinals would shrink by quite a few members.

Nevertheless, the O’Brien decision sets an historic precedent, said Terrence McKiernan of BishopAccountability.org, an online database of records on clergy abuse cases.

“It is a public demonstration of the role that clerics with inside information can have in bringing accountability to a church where secrecy has led to a crisis of sexual misconduct,” he said. “Cardinals who are tainted by the crisis cannot choose the person who will solve it.”

With O’Brien’s decision and the decision of a frail Indonesian cardinal to stay home, there are expected to be 115 cardinals under age 80 who are eligible to vote in the conclave.

Separately Monday, Benedict changed the rules of the conclave, allowing cardinals to move up the start date if all of them arrive in Rome before the usual 15-day waiting period between the end of one pontificate and the start of the conclave. It was one of his last acts as pope before resigning Thursday.