The Queen and the Duke of Edinburgh arrived today in Italy ahead of their first meeting with Pope Francis.
The royal couple flew into Rome for the one day visit, the first foreign trip by the Queen in more than two years.
They will attend a private lunch hosted by the President of Italy, Giorgio Napolitano, before entering the Vatican for a half hour private audience with the Pope.
The informal meeting with the head of the Roman Catholic Church will not be held in the Apostolic Palace at the Vatican but in the Pope's Study, part of a suite of rooms within the Pope Paul VI Audience Hall.
The hall is close to the Casa Santa Marta, the guest house in the Vatican where the Pope has chosen to live.
The Queen will not wear black or a mantilla - a lace veil - for the meeting in keeping with the informal nature of the visit, a Buckingham Palace spokeswoman said.
It is the Queen's seventh encounter with a head of the Roman Catholic Church. She previously met Pope Pius XII at the Vatican when she was Princess Elizabeth in 1951. She also met Pope John XXIII in 1961 and Pope John Paul II in 1980, 1982 and 2000. The meeting in 1980 made history after the Queen became the first British monarch to make a state visit to the Vatican. The Queen also met Pope Benedict XVI in Edinburgh at the start of his 2010 visit to Britain.
The Queen had been scheduled to visit Italy last year but the trip had to be postponed because she was unwell.
The visit - described as "part private, part official" - comes in the centenary year of the re-establishment of diplomatic relations between the Holy See and the British Government in 1914 after they were broken off in the 16th century.
The Queen, the Duke of Edinburgh and Pope Francis are expected to be assisted by an interpreter during the private audience as the pontiff is thought likely to speak in Italian or Spanish.
The Vatican Secretary of State Cardinal Pietro Parolin is expected to be present to greet the Queen at the Vatican on her arrival, along with his private secretary, Monsignor Robert Murphy.
Cardinal Cormac Murphy-O'Connor, the retired Archbishop of Westminster, and former leader of Catholics in England and Wales, will also be present in the group greeting the Queen and the Duke of Edinburgh on their arrival at the Vatican. The Cardinal, an old friend of the Pope, also has a close relationship with the royal family.
He has spoken of how the Pope asked him to convey his warmest greetings to the Queen just two days after his election last year.
It is believed that the 77-year-old Argentine-born Pope - who once described himself as from the "ends of the earth" - has never visited Britain.
Nigel Baker, British Ambassador to the Holy See, said: "The visit of her Majesty the Queen to the Vatican and the audience with Pope Francis is a reaffirmation of the strength of the bilateral relationship between the United Kingdom and the Holy See in a year in which we celebrate the centenary of the re-establishment of diplomatic relations with the Holy See."
The meeting between the Queen and the Duke of Edinburgh and the Pope has been described as a "private conversation". The Vatican is expected to issue a statement afterwards but it is not expected to give details of the discussions.
As Archbishop of Buenos Aires, the Pope attended a ceremony marking the defeat of Argentina in the Falklands War where he was reported as having described the Argentinian dead as having "set out to defend their mother, the homeland, to claim the country that is theirs and that was usurped".
But Vatican officials have made it clear that the Pope will remain neutral on the issue.
Asked to comment on the Falkland Islands, Mr Baker said: "The Vatican has been clear with us including in the last week and at a very senior level that their long standing position of neutrality on the issue remains in force."
The Queen was wearing a lilac, green and yellow floral chiffon dress by Stewart Parvin with a matching lilac coat.
She wore a lilac hat by Rachel Trevor-Morgan.
The Queen wore a large sapphire brooch with gold detail surrounded by diamonds.
Buckingham Palace said the brooch was acquired by Queen Mary in 1934 and bequeathed to the Queen in 1953.
The Queen was presented with a bouquet at the airport by the eight-year-old daughter of a British Embassy member of staff made of flowers from the garden of the British Ambassador's residence in Rome.
The Queen and the Duke of Edinburgh were greeted later at the Quirinal Palace by President Napolitano and his wife Clio.
The royal couple took the salute from a guard of honour made up of the Italian army, navy, the grenadier regiment of Sardinia and the Corrazieri, the presidential palace guard.
The Italian and British national anthems were played.
The royal couple then went into the Sala Del Bronzino within the palace where they were photographed with the president and his wife. They were expected to exchange gifts in private in the Studio alla Vetrata.
The royal couple will have lunch in the Sala Torrino, in the tower of the palace, a room with views across Rome.