The details of his poor health came as cardinals began the process of finding a new pontiff after the announcement on Monday that Benedict was to retire sent reverberations through the Catholic Church.
He will step aside completely from any role in running the Church after he formally resigns on February 28 and will play no part in the election of his successor, the Vatican said.
After his departure, Benedict will go first to the papal summer residence south of Rome and then to a cloistered convent inside the Vatican walls, exchanging the splendour of his 16th-century Apostolic Palace for a sober modern residence.
The Church's leadership said no specific illness led him to quit, merely old age and diminishing mental and physical strength.
Vatican spokesman Father Federico Lombardi added: "The Pope has said in his declaration that he will use his time for prayer and reflection and will not have any responsibility for guidance of the Church or any administrative or government responsibility.
"This is absolutely clear and this is the sense of the resignation."
Fr Lombardi added Benedict will not intervene in any way in trying to influence the choice of his successor.
As Scotland's 850,000 Catholics absorbed the news, Cardinal Keith O'Brien said: "We know something special has happened. This is the first time in 600 years that a Pope has resigned.
"But other work goes on as normal. For the ordinary Catholic going to Mass on Sunday, or not going to Mass, the work of the Church just goes on. Nothing stops. We just continue with the work and wait for a new man to give us leadership."
The Vatican is understood to be hurriedly rescheduling Pope Benedict's final weeks so that more people can see him before the resignation.
He was to have led a traditional Ash Wednesday service at a small church in Rome today, but the event has been moved to St Peter's Basilica for what will probably be his last Mass in public.
His final general audience, scheduled for February 27, has been moved from the Vatican's audience hall – which has a capacity of some 10,000 people –to St Peter's Square.
In the middle of next month, 115 cardinals will enter the Sistine Chapel for a conclave to elect the next leader of the world's 1.2 billion Catholics.
Informal consultations have begun about what type of leader the next Pope should be.
Potential successors include Brazil's Joao Braz de Aviz, 65, American Timothy Dolan, 62, Canada's Marc Ouellet, 68, and Gianfranco Ravasi, 70, of Italy.
l Music, painting, theatre, drama, poetry and art will be at the centre of this year's Lent festival in Glasgow.
The annual project, run by the Archdiocese of Glasgow Arts Project, will be launched today with details of its free opening concert, a special Gaelic concert and a travelling play about a pilgrimage to Lourdes.
The festival, directed by Stephen Callaghan, will run until March 31.