Seven Anglican priests and up to 300 members of several parishes are to join a new part of the Roman Catholic Church, a diocese has said.
The faithful, from six congregations, are to be welcomed into the Ordinariate - a grouping set up by the Pope for disaffected Anglicans.
The switch to Rome, reported to be the largest of its nature, involves three parishes in Essex and three in east London.
Between 250 and 300 churchgoers are expected to complete the move with them, a spokesman for the Catholic Diocese of Brentwood, in Essex, said. The transition follows the historic ordination of three former Anglican bishops as Catholic priests earlier this month.
The group of priests, including one retired vicar, met the Bishop of Brentwood, the Right Reverend Thomas McMahon, on Friday, ahead of their ordination.
"We had a wonderful day of sharing together and preparing for the future," Bishop McMahon said. "It is a marking moment for the life of our diocese."
However, the Anglican Bishop of Chelmsford, the Right Reverend Stephen Cottrell, expressed disappointed that members 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." It is unclear where the new congregations will worship.
The seven priests - from the parishes of Chelmsford, Hockley, Benfleet and Billericay, in Essex, Leytonstone, in London, and two from Walthamstow, also in London, will go through training before they are ordained as deacons in May and then as Catholic priests in June.
The Ordinariate was established for Anglicans who wished to join the Roman Catholic Church while retaining aspects of their heritage. The Vatican-approved scheme offers an alternative to opponents of women bishops, gay clergy and same-sex blessings.