Benedict will be joined by his older brother Georg, who is 88 and nearly blind, as well as the four Italian laywomen from a religious movement who have worked for him as housekeepers for years.

Benedict in February became the first pontiff to resign in more than 700 years in a move that stunned the world. A conclave of cardinals in March elected Pope Francis as his successor.

He has lived as a recluse in Castel Gandolfo some 25 kilometres (16 miles) south of Rome since February 28, never leaving the palace.

Benedict's secretary Georg Gaenswein, who is also head of the pontifical household for Pope Francis, is also expected to be present at the birthday,
which will be an entirely private event.

The pope spends his days praying, doing academic research and playing the piano in the palace on a hillside above Lake Albano, the Vatican said.

The Vatican last week denied that he had a serious illness, following rumours in the Spanish press.

In the most recent images of the former pope, he was seen walking slowly with the help of a cane.

The Vatican has said Benedict will eventually move to a former convent on a picturesque hilltop inside the Vatican walls to live out his days.