Seven Anglican priests and 300 members of six congregations are to join a new section of the Catholic Church, the Catholic Diocese of Brentwood says.
The move involves three parishes in Essex, and three in east London.
It is the largest known influx to date into the Ordinariate, which Pope Benedict established for Church of England members unhappy over issues such as the ordination of women.
Three former Anglican bishops have been appointed to lead the Ordinariate.
Ordinariates allow Anglicans opposed to developments including women bishops, gay clergy and same-sex blessings to convert to Rome while maintaining some of their traditions.
The Bishop of Brentwood, the Right Reverend Thomas McMahon, told BBC Essex the Anglicans were unhappy about the church's general move away from the traditions it once shared with Catholics, but described the decision as "a very big move".
"They relinquish their present post, a very big thing, leaving some of their people which brings heartache, into a fairly unknown future, as this ordinariate has only just been brought up.
"It calls for huge faith and huge trust because the future isn't that certain," he said.
Three vicars in Chelmsford, Hockley and Benfleet are among those men being trained to become Catholic deacons. A seventh retired Anglican vicar is also converting.
The Vatican will allow them to maintain a distinct religious identity and spiritual heritage within the Ordinariate.
The Anglican Bishop of Chelmsford, the Right Reverend Stephen Cottrell, said he was disappointed that 300 members in Essex were converting to Catholicism.
"Although I'm sorry these people are going, I do respect their decision," he told BBC Essex.
"But it is a small group of people. The Church of England remains the church for everyone."
According to a timetable set by the Roman Catholic bishops of England and Wales, former Anglican clergy and groups of worshippers wishing to enter the Ordinariate will be enrolled as candidates at the beginning of Lent in early March.
They will subsequently be received into the Roman Catholic Church and confirmed. This is likely to take place during Holy Week (17-23 April).
Where the new congregations will worship has yet to be decided.
"It will be on a case-by-case basis," said Father Keith Newton, the former Anglican bishop who now heads the Ordinariate.
"I hope in some cases the Church of England will be generous and there will be some sharing of Anglican premises. But I think normally our groups will be worshipping in Catholic churches," he added.
However, that does not mean that worshippers of the Ordinariate will be "mingled in" with Catholic congregations.
Funded by donations
"They will have a special service in their own right," said Bishop McMahon.
The Ordinariate will be funded initially by donations but its priests will not receive a salary, as they did in the Anglican church.
"We are hoping they will find some part-time work as chaplains in schools and hospitals," said Bishop McMahon. "We have already had some offers from charities."
Former Anglican bishops Andrew Burnham, Keith Newton and John Broadhurst were ordained into the group at Westminster Cathedral on 15 January.
At the time Father Newton estimated that about 50 Anglican clergy might join the Roman Catholic church - along with some members of their congregations.