By Robert Pigott
Religious affairs correspondent, BBC News
The nature of the news conference was kept secret beforehand
The announcement from the Vatican, made simultaneously in Rome and at a news conference in London was dramatic, even historic.
The Roman Catholic Church was going to extraordinary lengths to make it easy for disenchanted Anglicans to convert to Catholicism.
They could join the Roman Catholic Church as full members, but hang on to many of their Anglican traditions and practices - and indeed preserve much of their "Anglican identity".
In the past Anglicans have converted (although many have and are moving in the opposite direction), but it's been on a case-by-case basis.
The creation of a special section of the Roman Catholic Church - backed up by church law - especially for Anglicans all around the world is unprecedented.
At the somewhat bizarre press conference secretively arranged at the offices of the Catholic Church in London, the Archbishop of Westminster Vincent Nichols acknowledged converts had never before been provided with this structure.
"I can't remember quite a response that gives a juridical structure. And in that sense I think I would describe this as a courageous and a generous response by Pope Benedict."
It wasn't just that Rome is paving the way for traditionalists on the Catholic wing of the Church of England to jump ship - it is doing so at a critical moment.
But first a word of explanation - about why this move has such huge implications for the Anglican Church.
Since the Protestant Reformation, when the Church of England broke away from Rome, it has been a sometimes uneasy coalition between its Catholic and Protestant members.
It's the huge achievement of the Church that it has kept these two wings together through numerous crises - that over homosexuality being only one of the more recent.
But traditionalists on the Catholic wing have become increasingly disenchanted by "progressive" trends, not so much with respect to liberal moves on homosexuality, but about the ordination of women as priests, and, in the next few years, as bishops.
This development - utterly rejected by the Vatican for the Catholic Church - has been agreed by the Church of England Synod, and the only question is how far traditionalist parishes and clergy will be "sheltered" from having to serve under a woman bishop.
That debate is in the balance, and the Vatican's initiative is bound to have a profound effect - not just on the numbers who leave, but on the sort of church they leave behind.
Many traditionalist "Anglo-Catholics" have threatened to leave the Church and convert to Catholicism, and leaders of their cause say having a home already prepared for them will greatly increase the exodus.
Fr David Houlding, the leader of the Catholic Group on the Church's synod, said "several hundred" clergy would leave immediately, and something like 1,500 altogether.
Fr Houlding might have his own reasons for thinking big, but it does stand to reason that many wavering Anglicans, including married priests, will go and others will watch to see how they fare.
If they do leave in such numbers, the ground will be cut away from those left in the Church of England trying to preserve the Anglo-Catholic wing of this "broad church".
The departure of the most vociferous opponents of women bishops would surely reduce the pressure on the Synod to make concessions.
Some liberal Anglo-Catholics, who have no problem with women bishops but are desperate to preserve "catholic" traditions, fear they would leave behind a more Protestant church.
Other groups are also deeply unhappy about the way the Vatican sprang its idea.
Bear in mind that the Archbishop of Canterbury Rowan Williams knew nothing about this far-reaching move until two weeks ago, and made no contribution to it.
(Could it be a coincidence that at roughly that time news emerged that a special committee was suggesting more generous concessions to traditionalist opponents of women bishops?)
Some evangelicals - traditionalists but on the Protestant wing of the Church - have joined forces with Anglo-Catholics in an alliance resisting a number of "liberal trends".
Their targets have included the Church's approach to homosexuality, but also their joint opposition to the ordination of women as bishops.
Traditionalist evangelicals now stand to see an important ally massively weakened.
Their powerful lobby, the Fellowship of Confessing Anglicans, set up in Jerusalem last year, criticised the Pope's move as unnecessary, insisting that Anglo-Catholics had a home in their alliance.
Considering the audacity of the Vatican's initiative, it was muted criticism. But, off the record, evangelicals were briefing that Rome was capitalising on Anglican divisions to poach clergy.
Only last week the Vatican's senior spokesman on relations with other churches, Cardinal Walter Kasper, said "full visible unity" with Anglicans was Rome's long-term goal.
"We are not fishing in the Anglican pond," he insisted.
But by removing a potentially significant portion of the most "catholic" element from the Church of England, surely that sort of "reunion" has been set back.
Dr Williams stressed the Pope had only been responding to pleas for help from Anglo-Catholics, and insisted that this was not a hostile takeover.
"It has no negative impact on the relations of the (Anglican) Communion as a whole to the Roman Catholic Church as a whole."
However, the archbishop's representative in Rome, Bishop David Richardson, described the Vatican's move as surprising, asking "why this, and why now?"
The initiative, and the extraordinary way it emerged, also indicated the distance between the churches and the public they serve.
Journalists were called to a news conference, but officials refused to say what it was about.
Then the language in which the mysterious developments were explained would have struck most people as complete gobbledygook.
We learned that an "Apostolic Constitution" had been prepared, introducing a "canonical structure" which would establish "Personal Ordinariates".
These would allow former Anglicans to "enter full communion with the Catholic Church" while preserving their "spiritual and liturgical patrimony".
It gave the misleading impression of institutions that were out of touch and irrelevant to the lives of the many unattached but spiritually hungry people whom the churches need to attract.