A painting of the great Florentine ruler Lorenzo de' Medici by the Renaissance artist Raphael is expected to make up to £15million at auction.

The masterpiece was to be sent to Medici's intended bride in France.

One of the few Raphaels still in private hands, it is to be sold at Christie's in London in July by New York dealer Ira Spanierman.

He bought the work in 1968 when there were doubts about its authenticity. Three years later, research showed it was genuine. The wealthy Medici family were important art patrons during the four centuries they ruled Florence.

Pope Leo X arranged for Lorenzo, his nephew, to be married to Madeleine de la Tour d'Auvergne, a cousin of the King François I, in a bid to consolidate the family's position on the international stage.

An exchange of portraits was arranged because the couple had notmet. Richard Knight of Christie's said: "The importance of the artist and the sitter, together with the provenance and historical context behind this painting's creation, make it one of the most significant Old Masters to be offered at auction for a generation."

The portrait was recorded as being in the possession of Cosmo de' Medici, Grand Duke of Tuscany, in an inventory of 1553.

During the 19th century, it was owned by Lord Northwick before being sold at auction to Hollingworth Magniac, a merchant, in 1859. Magniac sold it for 567 guineas in 1892.

The painting will be on show at Christie's salerooms from 30 June to 5 July, the day of the Important Old Master and British Pictures sale.