The former Archbishop of Westminster, Cardinal Cormac Murphy-O'Connor, is seriously ill in hospital.
The church said the 84-year-old's health had taken a "defining turn".
Cardinal Cormac became the tenth Archbishop of Westminster in March 2000, was made Cardinal in 2001 by Pope John Paul II, before retiring from his role as archbishop in 2009.
As archbishop, he was the leader of the Roman Catholic Church in England and Wales.
Cardinal Vincent Nichols, the current Archbishop of Westminster, has asked for prayers.
In a statement on the diocese of Arundel and Brighton website, where Cardinal Murphy-O'Connor used to be bishop, Cardinal Nichols said "these loving prayers are a source of great strength and comfort as he calmly ponders on all that lies ahead".
Cormac Murphy-O'Connor was born on 24 August 1932 in Reading, Berkshire, as one of six children. Two of his brothers became priests and another played rugby for Ireland.
He was ordained priest in Rome in October 1956.
During an appearance on Desert Island Discs, he admitted that during his time training for the priesthood at the English College in Rome, he learned how to make a good Martini cocktail.
He returned to the college as rector from 1971-77 and on leaving was appointed as Bishop of Arundel and Brighton before taking up his role at Westminster Cathedral.