The strongly worded encyclical, sent to bishops around the world, raised fears that the pontiff may be hardening his line on inter-faith communion.

This is of particular concern to mixed-marriage couples in Britain and Ireland, and those wishing to see the church reunited one day.

However, theologians rushing to interpret the encyclical, the 14th of the Pope's 25-year pontificate, pointed out that he also underlined his earlier view that Catholics and non-Catholics could take communion together in special circumstances.

This was reassuring for those in Britain campaigning for more liberal interpretations of Catholic theology, at a time when the number of priests, as well as practising Christians, is in steep decline.

An important part of the ceremony of communion, or eucharist, is the blood and flesh of Christ being distributed to the confirmed in the form of wine and unleavened bread.

Catholics believe the bread and wine really does become the body of Christ when blessed by a priest, while for non-Catholics, the ceremony is merely symbolic.

The Pope said that Catholics "while respecting the religious convictions of these separated brethren, must refrain from receiving the communion distributed in their celebrations, so as not to condone an ambiguity about the nature of the eucharist and, consequently, fail in their duty to bear clear witness to the truth."

Many Anglican priests in Britain administer communion to Catholics, and vice versa, if a priest of the right denomination is not available or when couples of different denominations celebrate mass together.

While the Pope said it was not possible for Catholics and non-Catholics to celebrate communion together, he said he had "a burning desire" that one day this might change.

In a sympathetic response to the Pope's encyclical, Rowan Williams, the Archbishop of Canterbury, issued a statement saying: "I welcome the affirmation of the eucharist as a place of God's presence and action, and also welcome the Pope's reaffirmation of his 'burning desire' for common eucharistic celebration.

"This is an area of work which continues to be important for relations between Anglicans and Roman Catholics, and we continue to work theologically on this together."

Tony Blair, while still a member of the Church of England, regularly attends mass with his family, who are all practising Catholics.

In 1996, however, he was warned off receiving communion in a letter by the late Cardinal Basil Hume.

Saying that he had not realised his behaviour might cause offence, Mr Blair promised not to do so again, nevertheless adding: "I wonder what Jesus would have made of it?"

The prime minister and his family met the Pope at the Vatican during an official visit to Rome last month, and although he received a blessing - the first British prime minister to do so - he did not receive communion.