With rumour and intrigue swirling around the Vatican as cardinals prepare to elect a successor to Pope Benedict, the last thing the Roman Catholic Church needed was the ignominious departure last week of Cardinal Keith O'Brien.

The allegations against the cardinal are said to have been presented about a week before Pope Benedict announced his intention to step down on grounds of age and ill health – the same reason O'Brien gave for his own resignation last November, which the Pope originally said he would accept at some stage in the future, possibly on O'Brien's 75th birthday on March 17.

Creating further confusion, on the day it was announced that O'Brien's resignation had been accelerated, the Vatican published a statement saying the Pope had accepted the resignation on February 18 – a week earlier under canon 401, clause 1 of the church's Code of Canon Law.

That clause refers to the obligation of bishops to present their resignation on reaching their 75th birthday, and makes no mention of ill health or "other grave causes" that can also be invoked for a bishop's resignation.

Some Vatican observers said the swift and possibly retroactive acceptance of O'Brien's resignation as archbishop – he remains a cardinal with the right of voting in the conclave were he to change his mind and choose to exercise it – may have had more to do with a controversial BBC interview broadcast on February 22 in which he questioned longstanding Catholic teaching on priestly celibacy.

Vatican spokesman Father Federico Lombardi last week declined to comment on the affair.

"This is a nightmare for the Vatican," said Robert Mickens, Rome correspondent for the British Catholic weekly The Tablet. "There has been a constant drip of scandal connected to sexual abuse (separate to the O'Brien allegations) in the run-up to the conclave."

In his interview with the BBC's Glenn Campbell, O'Brien said the church's ban on married priests was "not of divine origin and it could get discussed again".

Vatican watchers said that view was likely to have caused particular anger in the curia, since O'Brien had taken a solemn oath to uphold traditional church teaching on priestly celibacy before being awarded his cardinal's red hat in 2003.

"This is a Pandora's box. They absolutely don't want to open up discussion of the possibility of married priests," said Mickens.

Catholic groups have been campaigning to convince cardinals who were negligent in overseeing paedophile priests to refrain from participating in the conclave.

The Italian media has been full of speculation about the allegedly lurid content of a report into corruption, careerism and sexual deviancy in the Vatican that was presented to Pope Benedict in December and may have influenced his decision to step down. Newspapers have spoken of a "gay lobby" in the curia, of Vatican officials fearful of blackmail by homosexual prostitutes, and of sexual rumours and innuendo being used to blight clerical careers.

Last week, the magazine Panorama claimed Vatican offices had been subjected to surveillance by the Vatican police service to try and discover the identity of people leaking confidential documents to the press. Phones and emails had been monitored and the number of CCTV cameras increased on the 44 hectares of Vatican City State, Panorama said.

Many clerics are said to be worried at what that surveillance may have produced and there is concern at how the contents of the report, known in full only to the Pope and its three authors, might influence the outcome of the conclave.

David Clohessy, a director of Survivors Network for those Abused by Priests (SNAP), is in Rome to lobby against the participation in the conclave of US cardinals, such as Los Angeles' Roger Mahony, accused of protecting abusive priests under their authority.

"We're grateful O'Brien is not at the conclave but it would have been better if the Pope had ordered him to stay home," Clohessy said.

The SNAP spokesman said it was rare for accusations such as those levelled against the cardinal to turn out to be false.

It was difficult to say whether the allegations against O'Brien, who might have been driven by a strong psychological compulsion, or those against Mahony, accused of a calculated decision to leave children at risk, were more egregious, Clohessy said.

"These are compromised men who have to consider the effect of their conclave vote on their own compromised careers," Clohessy said.

"We have to put the protection of the vulnerable and the healing of the wounded ahead of the reputation of an institution. Tragically, for decades, if not for centuries, church leaders have done the opposite."

Some have expressed suspicion at the timing of the media reports on the O'Brien allegations. One insider said he believed the Vatican may have known of the accusations since November, when the cardinal first submitted his resignation.

Archbishop Antonio Mennini, the papal nuncio in London, has clearly played an important role in the affair and has a past steeped in Vatican cold war intrigue.

His father, Luigi Mennini, was managing director of the Vatican Bank under Archbishop Paul Marcinkus at a time when the institution was deeply embroiled in the financial skulduggery of Banco Ambrosiano President Roberto Calvi and the anti-communist scheming of the P2 masonic lodge.

As a young parish priest in Rome in 1978, Mennini was chosen by Aldo Moro and his Red Brigades captors to carry messages to the kidnapped Christian Democrat's family.

"I am sure the role he played in the Moro affair was more significant than has so far emerged," said Sergio Flamigni, a former communist senator who served on parliamentary committees that investigated the kidnapping.

Mennini's intercepted telephone calls were tampered with and the young priest was posted as a diplomat to Uganda to keep him away from Italian judicial investigators, Flamigni said.

"He has always remained silent, not speaking to journalists or to the Italian authorities about his role. And he has been rewarded with positions of strategic importance in the course of a brilliant career," he said.

Some experts on the Moro affair suspect Mennini may have carried messages in both directions, even entering the politician's prison to hear his last confession before his execution by the Marxist terrorists.

Mickens, for one, saw the timing as fishy. "The allegations could have been leaked by Mennini to burn the English-speaking bloc. There's a lot of dirty politics around a conclave."