Pope Benedict XVI is stepping down after nearly eight years as leader of the world's 1.2 billion Roman Catholics.
The pope is due to meet cardinals before being formally bid farewell later by Cardinal Tarcisio Bertone and other members of the Vatican's Secretariat of State in Rome.
He is then due to fly by helicopter to Castel Gandolfo, the pope's summer retreat, with the empty office of pope, or the Sede Vacante, formally declared at 7pm British time.
The 85-year-old pontiff, who was elected in 2005 following the death of John Paul II, shocked the world earlier this month by announcing he was to resign on grounds of age and increasing frailty. He is the first pope to resign in nearly 600 years.
It was revealed that he is "grateful" for the prayers of British Catholics following the announcement that he is to step down.
A message from the Vatican addressed to the Archbishop of Westminster, the Most Rev Vincent Nichols, said the pope continued to give thanks for the "many graces" he received during his four-day visit to Scotland and England in September 2010. It added that Pope Benedict would pray for the UK faithful.
"He recalls with gratitude the welcome extended to him on his historic visit to your country in September 2010, and he continues to give thanks to God for the many graces received during those four days," the letter said.
"His Holiness promises to pray for all of you and for the people entrusted to your pastoral care, and he willingly imparts his apostolic blessing."
The outgoing pope held his last general audience in St Peter's Square, Rome, on Wednesday in front of a huge crowd. He recalled moments of "joy and light" but also times of difficulty when "it seemed like the Lord was sleeping".
Benedict XVI will be known as "Pontiff emeritus" or "Pope emeritus", after he has resigned and will keep the name of "His Holiness, Benedict XVI". He will dress in a white cassock without the mozzetta, or elbow-length cape, and will no longer wear the red papal shoes.