Pope John Paul II is expected to return to Russian authorities the Our Lady of Kazan icon stolen from St Petersburg in the early 20th century.
Vatican spokesman Joaquin Navarro-Valls on Saturday said the priceless 16th century icon would be handed back to the head of the Russian Orthodox Church on 28 August.
The icon has become something of a symbol of the pope's long-cherished intention to visit Russia, an event which has been blocked by strained relations between the Orthodox Church and the Vatican.
Navarro-Valls said the Pope hoped the return of the Madonna of Kazan would "contribute to the unity so longed for between the Catholic and Orthodox Churches".
Last year, the pontiff had planned a trip to Mongolia during which he hoped to stop over in Kazan, in Russia's autonomous republic of Tatarstan, to return the icon.
But Patriarch Alexy II – head of the Russian Orthodox Church which accuses Catholics in Russia of proselytising - opposed the plan.
Relations between the two churches have been sorely strained since Pope John Paul II announced in 2001 that he was creating four new Roman Catholic dioceses in Orthodox Russia, where Orthodox believers outnumber Catholics by more than 150 to one.
"A few weeks ago, the Holy Father communicated to the patriarch of Moscow his wish to give to the Russian Orthodox Church the sacred icon of the Lady of Kazan," Navarro-Valls said.
The date of 28 August has been agreed upon for the return of the icon because it marks the Feast of the Assumption of the Madonna in the Orthodox calendar.
The icon discovered in Kazan in 1597 is venerated for its miraculous powers. It was stolen from the cathedral at Kazan in 1904 and taken abroad.
After it was stolen from St Petersburg, the icon is understood to have been bought from a dealer by a Catholic organisation and presented to the Pope in 1993. Since then, it has been hanging in his private rooms at the Vatican.