Tens of thousands of disaffected Anglicans could become Roman Catholics following a decree made today by Pope Benedict to poach whole Protestant communities for the church for the first time since the Reformation.

The move surprised many as it was unveiled simultaneously in the Vatican and in London, where the Archbishop of Canterbury, Rowan Williams, was forced to admit that he had not known about it until a fortnight ago.

Pope Benedict's initiative – set out in an apostolic constitution, the highest form of pontifical decree, and unveiled by a senior Catholic cardinal – allows Anglicans worldwide, both clergy and worshippers, to convert en masse while still maintaining part of their spiritual heritage.

Both Williams and the Catholic archbishop of Westminster, Vincent Nichols, who was sitting next to him in a show of unity, refused to concede that the Vatican was passing judgment on the troubles within the Anglican communion.

"It is not an act of aggression, it is not a statement of no confidence. It is business as usual," said Williams, who nevertheless apologised to Anglicans that there had not been a chance for prior debate. The Vatican sought no input from Lambeth Palace.

The papal decree comes after many years of approaches to the Vatican from Anglicans unhappy with the ordination of women and gay people.

There was scarce detail about how the new structure would work – there could be separate services in Roman Catholic churches for breakaway Anglicans who could organise their own bishops and create their own prayer books, though ultimate control would lead back to Rome. It creates not so much a church within a church as an enclave operating under the auspices of the Vatican or a safe haven for endangered Anglicans.

The most significant part of the decree is that it will allow married Anglican clergy to be ordained as Roman Catholic priests, waiving the requirement of celibacy.

The pope's chief theological adviser, the US cardinal William Levada, said that he would put the number of Anglican bishops in the world who were poised to become Catholics "in the 20s or 30s". Later, Joseph Di Noia, the deputy head of the Vatican's liturgical department, said he believed the figure was closer to 50.

Williams today appeared alongside Archbishop Nichols in Eccleston Square, the administrative headquarters of the Catholic church in England and Wales, and there were awkward moments. When Williams was asked if the Vatican move was a "massive vote of no confidence" in his leadership, it was Nichols who jumped in with an answer. Several times they both said the apostolic constitution was not a commentary on the internal disputes ravaging the world's third biggest Christian denomination – despite years of Roman consternation over the ordination of women and gay people.

At an Anglican conference last year several cardinals swooped into Canterbury to air their concerns about the impact such innovations would have on relations between the two churches and how undesirable an Anglican schism would be.

But today in a basement room, faced with the press, Williams was optimistic and resolute, though his complexion reddened. "I do not think this constitution will be seen as in any sense a commentary on Anglican problems offered by the Vatican. It is a response to this range of requests and inquiries from a very broad variety of people. In that sense it has no negative impact on the relations of the communion as a whole to the Roman Catholic church as a whole."

There were no "regular or routine channels of communication" with the Vatican department that made the decision – the congregation for the doctrine of the faith - Williams explained, because his relationship was with the Pontifical Council Promoting of Christian Unity. It is unknown whether this body was aware of the constitution prior to its unveiling but chose not to tell him.Williams was also forced to reveal his ignorance about the move to Anglican bishops and archbishops, a number of whom are dissatisfied with his leadership. News that he was kept in the dark will only reinforce their antipathy towards him.

In a letter 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, and we await the text of the apostolic constitution itself and its code of practice in the coming weeks."

Two bishops from a prominent Anglo-Catholic movement in the UK, Forward in Faith, welcomed the apostolic constitution, but said it was not a time for "sudden decisions or general public discussion".

The bishops of Ebbsfleet and Richborough, who provide pastoral and spiritual care for people opposed to women bishops, also confirmed their 2008 meeting with Vatican officials, an event previously denied by Lambeth Palace.

They said some would want to stay in the Anglican communion, while others would make arrangements according to their conscience. They said they had chosen 22 February "to be an appropriate day for priests and people to make an initial decision as to whether they wish to respond positively" to the apostolic constitution.

"We were becoming increasingly concerned that the various agendas of the Anglican communion were driving Anglicans and Roman Catholics further apart. It was our task, we thought, to take the opportunity of quietly discussing these matters in Rome. We were neither the first nor the last Anglicans to do this in recent years. Following the decision of the General Synod of the Church of England in July 2008 to proceed with the ordination of women … we appealed to the Holy Father and have patiently awaited a reply."

The initiative is not without problems for the Vatican. The Catholic church will accept married Anglican vicars who agree to be reordained, just as it includes married priests of the so-called Uniate churches that belong to the Orthodox rite. But, like the Orthodox, it draws the line at married bishops.

However, under the arrangements Anglicans can be taken into so-called "personal ordinariates" in each country, similar to military chaplaincies. Each would be headed by a former Anglican prelate, who does not have to become a bishop in the Catholic church, and so could be married. By accepting numbers of married clergy, some with the responsibilities and status of bishops, the Vatican risks reigniting the debate among Catholics over its insistence on celibacy for the vast majority of its priests who belong to the western, or Latin, rite. The latest move will also revive unease in the Orthodox world where the Vatican is under constant attack for allegedly trying to poach adherents.

The reverberations of the yesterday's announcement will be felt for some time to come. It will seriously test the diplomacy skills of the Queen, the supreme governor of the church of England, and Williams, when they receive the pope in 2010 for his first official visit to the UK. It will also affect general synod as it grapples with legislation on women bishops and the election of a new membership.

Rewriting the rule book

Will breakaway Anglicans get their own churches? The details are still to be decided, but there may be separate services held in Roman Catholic churches. There may also be special prayer books and training centres for them. The chain of command will still lead to the pope (pictured right).

Will these Anglicans be 100% Roman Catholic? They have to accept all Roman Catholic doctrine and teaching. But they could use elements of Anglican tradition. The big news is that married clergy can be ordained as Roman Catholic priests, with a view to eventually providing pastoral care for other former Anglicans. Celibacy is normally a requirement to join the Catholic priesthood, but this new global structure has torn up the rule book.

Why do people want to leave the Anglican Communion? Two reasons: women and gay people. For the last few decades traditionalists have been unhappy with the liberal progression of the church.