ANKARA, Turkey -- Bartholomew I, the spiritual leader of the world's Orthodox Christians, is heading to the Vatican to attend Pope Francis' installation Mass -- the first time an ecumenical patriarch is attending a papal investiture since the churches split nearly 1,000 years ago, a church official said Monday.

The Roman Catholic and Orthodox churches were united until the Great Schism of 1054, which was precipitated largely by disagreements over the primacy of the pope.

Francis' predecessor, the now-retired Pope Benedict XVI, had made uniting all Christians and healing the split a priority of his pontificate. A joint committee has also been working to mend the rift between the two branches of Christianity.

Rev. Dositheos Anagnostopoulos, the spokesman for the Istanbul-based Patriarchate, said Bartholomew would become the first Orthodox spiritual leader to attend an investiture since the Schism. The decision to attend the Mass at St Peter's Square on Tuesday was "the fruit" of the growing dialogue between the Roman Catholic and Orthodox churches, he said.

Bartholomew has made several previous visits to the Vatican, including attending the funeral of Pope John Paul II in 2005. Bartholomew also hosted Benedict during a 2006 visit to Istanbul, the sprawling city formerly called Constantinople and the ancient spiritual centre of the Orthodox churches.

Prof. Ionnis Zizioulas, the Metropolitan of Pergamon and a co-chair of the joint commission promoting dialogue between the two churches, is accompanying Bartholomew to Rome, Anagnostopoulos said. Bartholomew's delegation will also include Tarassios, the Metropolitan of Argentina, and Gennadios, the Metropolitan of Italy.

Francis is familiar with Orthodox traditions from 14 years of heading the Argentine church's commission on Eastern Rite Christians, who are within the Catholic fold but follow Orthodox religious customs, including some married clergy in lower ranks.

The powerful Russian Orthodox Church, the largest of the more than dozen Orthodox churches,welcomed the election of Francis, the first Latin American pope, as a step toward better relations, which have steadily improved over the past decades.

Francis was expected to meet with representatives of other religious denominations on Wednesday.