Pope Benedict XVI has just announced his intention to allow Anglicans to convert in groups to the Roman Catholic Church. Whole congregations would be allowed to move to Rome, while retaining elements of Anglican spiritual and liturgical practice.

This is a significant change. Those who left the Church of England in the past, such as those who would not accept the ordination of women, were told that they would have to become Catholics as individuals. Now Anglicans will be allowed to join as groups. They will even be allowed their own church within a church, called something like the Anglican Ordinariate but which would be subject to the discipline of Rome. Some fear that this will decimate the Church of England.

What changed?

For years Catholic bishops in England were reluctant to open the door wide to traditionalist Anglicans, partly because their "more Catholic than the Pope" smells-and-bells churchmanship was out of step with modern Catholicism. They also did not want to upset Church of England bishops with whom they had developed strong ecumenical relations.

But then last year the Church of England's General Synod voted to allow women to become bishops. More importantly it also voted for no special provision to be put in place to allow traditionalists to bypass a woman bishop and seek episcopal oversight from a man. The decision radically shifted attitudes in Rome.

So whose initiative is this?

It began with Anglican dissidents for whom women bishops were the final straw but who were already alarmed by women or gay priests.

Two years ago an Australian archbishop, John Hepworth, leader of the Traditional Anglican Communion which claims to represent 400,000 worshippers worldwide, went to the Vatican to seek terms for his flock to be accepted into full communion with Rome. Part of the Roman Curia received him sympathetically, but the dominant group of Vatican bureaucrats were against him.

But then came the Synod vote. After it, six Church of England bishops approached the Vatican and said they were being frozen out of the Anglican communion. They pleaded for some sort of structure to be created inside the Catholic Church for their wing of Anglicanism.

So is Rome being generous – or predatory?

The spin is that the Pope is just being helpful. The Archbishop of Canterbury, Dr Rowan Williams, wrote to his bishops to say that the move was not "an act of proselytism or aggression" or "at all intended to undermine existing relations between our two communions". But when he and the Catholic Archbishop of Westminster, Vincent Nichols, appeared together to launch the initiative, both men looked uncomfortable.

In Rome the announcement was made by Cardinal William Levada, Prefect of the Congregation for the Doctrine of the Faith – the Vatican's doctrinal watchdog. Cardinal Walter Kasper, the President of the Pontifical Council for Promoting Christian Unity, was noticeable by his absence. The word was that the Vatican's leading ecumenists had fought the move behind the scenes – and lost.

Is this a snub for Rowan Williams?

Certainly the Archbishop was caught unawares by the move. Until now Rome has privately taken the view that it did not want to undermine his attempts to maintain Anglican unity. They even sent, unprecedentedly, three cardinals to support him at the last Lambeth Conference. Cardinal Kasper told Anglican bishops that they had to choose between being a church in the first-century apostolic tradition, or one in the 16th-century reformed tradition.

Rome decided that the Synod vote had answered that question decisively. Anglicanism was a lost cause. Vatican officials told Dr Williams about their plan just three weeks ago. Cardinal Levada came only last weekend to spell out the detail of the radical proposal to him. Publicly Dr Williams insists that the Pope's plan was in no way a "commentary on Anglican problems". Privately he is said to be rather cross.

What does it mean for Anglicanism?

Some think it bad news. Evangelicals are already creating a major split in the Anglican Communion over liberals' embrace of gay priests. The Pope's plan will allow traditionalists to defect in the other direction, leaving the Archbishop of Canterbury and a dwindling centrist core with an ever more difficult job to fend off calls for disestablishment from increasingly aggressive secularists.

But other Anglicans are delighted. They argue that it will mean the CofE can stop tying itself in knots trying to make concessions to traditionalists. They can be told, accept the Anglican way or shove off to Rome. Some hardline liberals even want to see the dissident bishops disciplined over their negotiations with Cardinal Levada, which they say breached the oath of obedience they took at their consecration. And there have been calls for the Queen to refuse to meet the Pope and for plans for next year's papal visit to England to be scrapped.

Probably, compromise being the Anglican genius, nothing so extreme will happen either way.

How many people will leave?

It is in many people's interests to big this up. There has been talk of as many as a thousand CofE priests leaving, plus thousands more in America and Australia. The 1,000 figure comes from the church's traditionalist Forward in Faith faction (whose critics call it Backward in Bigotry).

But how many will carry out the threat?

When women priests were first ordained it was said 1,000 priests would quit. In the event only 441 took the financial compensation package on offer, and scores of those have returned from Rome disillusioned since.

The main movement will probably not be in England. Of the 50 Anglican bishops who have made tentative approaches to Rome, only half a dozen are from England. Rome is taking more notice of the concerns of the US rather than UK Catholic bishops on how to handle all this.

As a counterbalance, women priests who were unhappy at the prospect of second-class female bishops will now probably withdraw their threats to leave, though they numbered only dozens.

What does it signify for Rome?

That's hard to read. It may want a separate Anglican Ordinariate in order to quarantine the newcomers from cradle Catholics. Rome doesn't want the influx of married priests to add legitimacy to the call for married priests among mainstream English Catholics. But it may also reflect the emerging difference in style between Pope John Paul II, who loved to do battle with those who disagreed with him, and his successor, who prefers to ignore his mainstream liberal critics and instead promote those who unquestioningly accept his view of the church and the world.

On Monday Vatican officials begin talks with the extreme Lefebvrist traditionalists in the Society of Saint Pius X, whose Holocaust-denying bishop the Pope controversially readmitted to the church earlier this year. The structure of an Anglican Ordinariate might offer a useful precedent for their readmission to the bosom of Rome.

Has the Pope overstepped the mark?

Yes...

* He sprang his plan on the Archbishop of Canterbury with no consultation and hardly any warning.

* The Vatican officials responsible for Christian Unity tried to block the move – but they were over-ruled.

* He could have stuck to the policy of welcoming individuals but he is now trying to entice whole congregations.

No...

* The Archbishop of Canterbury has said the Pope's move is not "an act of proselytism or aggression".

* Rome is merely welcoming those the General Synod has frozen out.

* The Pope gave Anglicans every chance to back the Archbishop of Canterbury's quest for unity, but they failed.