As many as 1,000 priests could quit the Church of England and thousands more may leave churches in America and Australia under bold proposals to welcome Anglicans to Rome.

Entire parishes and even dioceses could be tempted to defect after Pope Benedict XVI’s decision to offer a legal structure to Anglicans joining the Roman Catholic Church.

His decree, issued yesterday, is a serious blow to attempts by the Archbishop of Canterbury, Dr Rowan Williams, to save the Anglican Communion from further fragmentation and threatens to wreck decades of ecumenical dialogue.

Dr Williams was notified formally only last weekend by the Vatican and looked uncomfortable at a joint press conference with the Archbishop of Westminster, the Most Rev Vincent Nichols, to announce the plan.

Times Archive, 1845: Secession of the Rev Newman

In every point of view this movement must be the subject of deep regret

• The Rev Mr Newman, 1843

• Leading article: Cardinal Newman, 1890

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Anglicans privately accused Rome of poaching and attacked Dr Williams for capitulating to the Vatican. Some called for his resignation. Although there was little he could have done to forestall the move, many were dismayed at his joint statement with the Archbishop of Westminster in which they spoke of Anglicans “willing to declare that they share a common Catholic faith and accept the Petrine ministry as willed by Christ for his Church”.

In a letter to bishops and clergy, Dr Williams made clear his own discomfiture. He wrote: “I am sorry that there has been no opportunity to alert you earlier to this. I was informed of the planned announcement at a very late stage.”

The Bishop of Fulham, the Right Rev John Broadhurst, chairman of Forward in Faith, which opposes women bishops, hailed it as a “decisive moment” and predicted that, based on his group’s membership, up to 1,000 Church of England clergy could go.

Christina Rees, of the pro-women group Watch, described the Vatican’s move as poaching. She said: “It is one thing to offer a welcome, but this seems to be a particularly effusive welcome where people are almost being encouraged. In the Anglican Church we like to operate with transparency. If this has not been done here that will add to the sense of this being a predatory move.”

Pope Benedict wants to make Christian unity an enduring legacy of his papacy. He is due to visit Britain next year; Dr Williams will visit Rome next month. The Pope has already shown his determination to reunite Christendom at almost any price, welcoming back the traditionalist Society of Saint Pius X despite a Holocaust-denying bishop in its ranks.

Under the plan, the Pope will issue an apostolic constitution, a form of papal decree, that will lead to the creation of “personal ordinariates” for Anglicans who convert to Rome.

These will provide a legal framework to allow Anglicans to enter full communion with the Catholic Church while preserving distinctive elements of their Anglican identity, such as liturgy. Clergy will have to be retrained and re-ordained, since Rome regards Anglican orders as “absolutely null and utterly void”, but they will be granted their own seminaries to train future priests for the new ordinariate.

This deal was done with the Congregation for the Doctrine of the Faith, formerly the Holy Inquisition that Cardinal Joseph Ratzinger himself headed before he became Pope.

The Council for Christian Unity was not represented at simultaneous press conferences in Rome and London, suggesting that the Pope has had enough of dialogue focusing on canonical moves towards unity. Dr Williams was briefed formally only when Cardinal William Levada, of the Congregation for the Doctrine of the Faith, flew to London at the weekend to tell Anglican and Catholic leaders of the plans. It is understood that leading members of the council and other senior Anglican and Catholic figures tried desperately to block the decree.

One result of the Vatican’s move is that women bishops are likely to be consecrated sooner rather than later in the Church of England. This is because Parliament and the General Synod will not sanction legal structures to “safeguard” opponents of women priests within the Church if Rome is offering an open door with the Archbishop of Canterbury’s blessing.

Dr Williams said that the announcement did not disrupt “business as usual” in relations between the two churches. It would be a serious mistake to view the development as a response to the difficulties within the Anglican Communion, he said. It was aimed at people who had reached a “conscientious conviction that visible unity with the Holy See was now what God was calling them to”, he said. “It is not a secret that in this country the ordination of women as bishops is one of those test issues.”