Just months after its publication in July the Vatican is to issue a "document-instruction" to clarify details of the Pope's recent motu proprio liberalising the use of the Tridentine Mass. Mgr Camille Perl, Secretary of the Pontifical Commission Ecclesia Dei, indicated last month that such a document would clarify the "right interpretation" of the motu proprio and answer criticisms and doubts. The existence of the "clarifying" document was confirmed this week by other sources.

One of the most common questions following the publication of the motu proprio concerns the number making up the "stable group" required for the celebration of an old rite Mass. While the document is expected to clarify that question, other issues are left unresolved. Leading Jesuit Keith Pecklers, Professor of Liturgy at the Gregorian University in Rome, said that the prayer for the conversion of the Jews in the Good Friday liturgy, "given the great progress made in Jewish-Christian relations, remains quite problematic - that is a pitfall that was not sufficiently anticipated".

In addition, the seeming coexistence of old and new rites "doesn't work", Professor Pecklers said, particularly regarding Sacraments of Confirmation and Ordination. "What do you do if you have a number of priests who want to be ordained in the old rite? What does it suggest of the substantial unity of the Church?" The main issue, Professor Pecklers said, was not one of liturgy but of ecclesiology. "The proponents of the Tridentine Rite have never even partially accepted the view of the Church of the Second Vatican Council - the view of the laity, the opening up of the Church to the modern world, ecumenical dialogue, and interreligious outreach to Muslims and Jews," he said.

John Medlin, general manager of the Latin Mass Society of England and Wales, said he thought that an explanatory document from Ecclesia Dei was due within a month. Meanwhile sources in Rome confirmed that such a document is under preparation, and added that it may respond to Jewish concerns about prayers for the conversion of the Jews, included in the old rite liturgy.

This was also indicated by the Israeli ambassador to the Vatican, Oded Ben Hur, who said he expected the clarifications to be published by Easter "to solve the problem before it erupts". Mr Ben Hur said the motu proprio had prompted Israel's chief rabbis to send a letter to the Pope expressing their concern. The Good Friday litany in the old rite includes a prayer for the conversion of Jews that "God may remove the veil from their hearts".

Cardinal Tarcisio Bertone, the Vatican Secretary of State, and Cardinal Walter Kasper, President of the Pontifical Council for Promoting Christian Unity, "have promised to examine it and the text will be changed", the ambassador said, although he did not know the wording of the proposed new prayers.

There is speculation that Pope Benedict XVI might officiate at a Tridentine Rite Mass on 9 December in the Basilica of St Paul's Outside the Walls, in Rome. Such a choice, if confirmed, would be highly symbolic, because it was in this church that Pope John XXIII announced his decision to call the Second Vatican Council, which produced the new rite.

Last week a senior Vatican official rebuked bishops who had tried to limit the use the old rite in their diocese. Archbishop Malcolm Ranjith, secretary of the Congregation for Divine Worship, said such bishops were in rebellion against the Pope and guilty of pride