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 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.