The document is expected, among other things, to reaffirm the ban on artificial contraception which caused a crisis in the Roman Catholic church in the developed world when restated 25 years ago in the encyclical Humanae Vitae.
According to reports in the Italian press, the encyclical Veritatis Splendor (the Splendour of the Truth) has been completed after a long gestation period, and will be published at the end of this month.
Sections of an earlier draft were published in Italian papers at the weekend and suggested that the encyclical will take a very hard line on the issue of papal authority, which many Roman Catholics believe has damaged the papacy far more than what is actually taught.
The Catholic World Report, a right- wing Catholic news magazine with an excellent record for accuracy, published in its latest issue an account of the encyclical letter suggesting that it is largely aimed at clamping down on moral theologians and philosophers.
Well-placed observers within the English Catholic church suggested that the encyclical is not expected to concentrate on sexual morality. Rather, it restates the papal claim to a supernatural discernment in matters of morality generally. As such, it is likely to cause greater ructions among the celibate clergy.
In crude terms, the battle lines are being drawn between an 'objective morality' - according to which certain acts such as abortion are always and intrinsically wrong - and a 'subjective morality', which enables liberal moralists, by taking into account motivation, circumstances, and likely results of an act, to conclude that it was not so harmful after all.
The Catholic World Report claims that the argument of the encyclical concludes with the assertion that 'moral theologians must teach the doctrines of the church and not their own personal theories'.
However, in most of Western Europe and North America, it is almost impossible to find a Catholic moral philosopher who privately believes in the ban on artificial birth control. It is certainly difficult to find any Catholic lay people who do, and the birth rate among middle-class Catholics is exactly the same as among middle- class agnostics.
John Wilkins, the editor of the Tablet, an English weekly Catholic news magazine, says that it is reactionary teaching on authority, rather than on particular sexual practices, which has really harmed the church. In the developed countries, lay people resent being talked down to, 'but even in the developing countries, what they want is evidence that they are being listened to.'
The account in the Catholic World Report says that the real enemy of Catholic truth is a doctrine of liberty now taught in many Catholic colleges and seminaries, which exalts conscience above Church teachings.
However, the encyclical is reported to argue that 'the Law of God and man's true freedom cannot be in conflict, since both are ordained and desired by God'.
'The Church does not deny the freedom of the Christian conscience. But the conscience must be properly formed, free from ignorance and
error,' according to the Catholic World Report.
There has been speculation in this country and elsewhere that the encyclical has been deliberately leaked in order to discredit it before publication, and possibly even to prevent the Pope from signing it - though such a gesture would be unprecedented.
There has been as yet no official comment on these speculations. But both supporters and opponents believe that the encyclical is very authoritarian.
Leading article, page 15