THE participation of Britain's most senior Catholic cleric in the selection of the next Pope has been cast into doubt amid allegations of inappropriate behaviour towards young priests dating back to the 1980s.

A wall of silence last night surrounded Cardinal Keith O'Brien following claims three priests and one former member of the clergy have made complaints about the cardinal to the Pope's ambassador to the UK, with his role in the conclave now being questioned following the allegations.

The Pope has been informed of the claims, with a Vatican spokesman confirming written allegations against Cardinal O'Brien were being studied by the pontiff, saying: "The issue is now in his hands."

Cardinal O'Brien, the head of the Scottish Catholic Church, missed giving mass at his cathedral yesterday to celebrate the Pope's final appearance before standing down, citing legal advice. He contests the allegations.

Auxilliary Bishop of the Archdiocese of St Andrews and Edinburgh, Stephen Robson, said Cardinal O'Brien was very upset by the claims and a statement may be issued today.

Bishop Robson told the congregation at St Mary's Cathedral he had a "heavy heart" as he addressed them.

He said: "A number of allegations of inappropriate behaviour 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."

A number of prominent Catholics have questioned the timing and nature of the allegations and the potential

impact on the conclave, while conservative critics of the cardinal claim they had been presented with allegations regarding him over the years.

Cardinal Cormac Murphy-O'Connor, the former archbishop of Westminster, who will be travelling to the Vatican to advise cardinals voting for the next Pope, said he was very sad to hear about the allegations.

Asked whether the cardinal should take part in the conclave, he added: "That is up to Cardinal O'Brien to decide, and I think rightly so.

"The allegations have not been proved in any way, so he will have to decide whether he wants to go."

Cardinal O'Brien, who will be the only British representative at the conclave, has been an outspoken critic of gay rights.

He is due to retire next month when he will be 75, after having taken part in the conclave.

On Friday he again dominated the global headlines with claims that he was supportive of the idea of priests being married.

It has been reported that three priests and one former priest, from the diocese of St Andrews and Edinburgh, complained to the Papal Nuncio, Antonio Mennini, of what they alleged was the cardinal's inappropriate behaviour towards them in the 1980s. The four complained the week before February 11, when Pope Benedict announced his resignation. The complainants have called for the cardinal's immediate resignation.

One former priest, who is now married, claims Cardinal O'Brien made an inappropriate approach to him in 1980, after night prayers, when he was a seminarian at St Andrew's College, Drygrange.

A second complainant's statement says he was living in a parish when he was visited by O'Brien, and inappropriate contact took place between them, while the third alleges dealing with what he describes as "unwanted behaviour" after late-night drinking.

The fourth complainant claims the cardinal used night prayers as an excuse for inappropriate contact.

John Haldane, Professor of Philosophy at St Andrews University and an authority on the Catholic Church, said: "The accusations that have been reported are unsettling and their appearance at this point in the week when the Pope abdicates, and while the Church and the world are focused on the impending conclave, is bound to be troubling."

Dr Jenny Wormald, one of the most influential historians of early modern Scotland and a prominent lay Catholic, questioned why the complaints were not aired ahead of the 2005 conclave, adding that the ex-priest no longer had to worry about the Catholic hierarchy.

She added: "I hope it doesn't have an input into his role at the conclave and he's got to make his own decision on attending. You might say all cardinals are sinners. Should they stay away?"

Another lay Catholic said much about the allegations "didn't stack up", implying the cardinal was being targeted.