By BBC News Online's Peter Gould

The increasing frailty of Pope John Paul II has heightened speculation that he may be forced to stand down.

Honduran cardinal Archbishop Oscar Maradiaga has fuelled the debate by telling reporters in Italy that the Pope had not ruled out stepping down if his health deteriorates further.

Once elected, a pope usually remains in office until the day he dies.

But there is now concern that the 81-year-old pontiff may be finding it difficult to perform his duties.

There is no procedure for dealing with a pope who becomes incapacitated

Thomas Reese Vatican expert

A papal resignation would trigger a conclave - a secret meeting of cardinals who would elect his successor.

A few popes have abdicated in previous centuries. The last to do so voluntarily was Celestine V in 1294.

Under modern canon law, a pope is allowed to resign.

But John Wilkins, editor of Catholic newspaper, The Tablet, said: "He would say that it is a job given to him by God, and he has no right to lay it down."

Painful

The physical decline of John Paul II has become painfully obvious to those who have seen his recent public appearances.

Father Thomas Reese: Church faces a crisis

At Easter, he was unable to take part in some of the traditional ceremonies - the first time he has been forced to pull out since he was elected in 1978.

The Vatican does not deny that he is suffering from a form of Parkinson's disease.

He also has arthritis, which has become increasingly painful in recent months.

Yet most Vatican observers are convinced it is the Pope's wish to continue in the job until his death, and resignation would be a last resort.

"I think he is probably going to be in office until the day he dies," says Father Thomas Reese, an authority on the workings of the Vatican, and editor of the Catholic magazine America.

"Remember, this is a Pope who loves the Church and wants to serve it as long as he can. But if he comes to the conclusion that for the good of the Church he needs to step aside, then I think that is what he will do."

Coma

It is rumoured that the Pope has anticipated such a scenario, and has left instructions on what the Vatican should do.

This could cause a major constitutional crisis because medical technology today can keep someone alive way beyond the point where they are capable of functioning

Thomas Reese Vatican expert

"There is a story that the Pope has written a letter which says that if he becomes unable to exercise his faculties, his papacy must be considered to be at an end," said John Wilkins.

"I believe it is quite possible that there is such a letter.

"There is no provision for a pope who becomes incapable and disabled, and does not know where he is. There is nothing you can do. You have to go on managing the Church with a number two or number three, or a group, until the pope dies."

Father Thomas Reese, an authority on the government of the Church, agrees that the Vatican should be prepared for such an eventuality.

"We do have a serious problem," he says.

"Suppose the pope goes into a coma, or suppose a pope becomes mentally unbalanced, or suffers from Alzheimer's disease.

Legislation

"If it gets to that kind of a situation, where the pope is very sick and can't function, but he is not even well enough to be able to resign, then we don't know what to do.

John Wilkins: Pope may have left a letter

"In the Catholic church there is no procedure for dealing with a pope who becomes incapacitated.

"So this could cause a major constitutional crisis, especially because medical technology today can keep someone alive way beyond the point where they are capable of functioning."

Father Reese believes that in such an eventuality, the Church could find itself powerless to replace the pope.

"I think it would pray and hold its breath and probably just wait for him to die," he says.

"There are some canon lawyers who have come up with theories on ways the Church could handle this, but there are differences of opinion.

"What is really needed is for the Pope himself to promulgate legislation which would say how the Church should deal with this.

"But nobody in Rome really wants to talk about that kind of thing, let alone suggest it to the Pope."