The angel that turned into a Renaissance beauty

But underneath Tobias lay an entirely different painting altogether. And the work that emerged after a slow and painstaking 20-year restoration is a stunning portrait of a lady and her daughter that was indubitably by the Venetian old master Tiziano Vecellio, better known as Titian.

Bidding for Tobias and the Angel stopped at £2,300 in 1963, the last time it came up for public auction, failing to make its reserve price. But Portrait of a Lady and her Daughter is expected to fetch at least £5m at Christie's old masters sale on 8 December.

Francis Russell, deputy chairman of Christie's, said: "This represents an opportunity to buy a portrait of great beauty and a certain mystery and of a very high calibre."

Women rarely feature in Titian's many portraits and this work is the only one known to show a mother and daughter. They may be his daughter, Emilia, with one of his granddaughters.

The painting was probably begun in the 1550s but set aside by the artist within days - possibly because it was not a commission and there was no financial imperative to complete.

After Titian died in 1576, it appears that a pupil reworked the canvas for sale. The woman was given wings and became the Archangel Raphael, while her daughter was given a male hairstyle to become Tobias.

The work was first recorded in the mid-18th century in the famous collection of a Venetian, Gregorio Barbarigo, who owned many works that had come straight from Titian's studio after his death.

The Barbarigo collection was sold in 1850 to Nicholas I, and went to the Hermitage in St Petersburg. When the tsar disposed of several thousand works, this unfinished Titian was sold to Count Tyszkiewicz, a Russian aristocrat.

By the 1920s, it was in the hands of Rene Gimpel, a renowned French picture dealer, who sent works to London when war broke out in 1939. He joined the French Resistance and died in a slave-labour camp, having told no one of their location.

They were found in a garage in Bayswater, west London, in 1946. The work went back to family members in France, who decided it should be restored, but it is now owned by an unnamed company.

Francis Russell said restoration had been a risk. "You could have been wrecking one picture to find something that wasn't worth finding," he said.

Rene Gimpel, a London art dealer who is the grandson of his namesake, said the family "had just had a hunch that there might be more of a Titian under the Titian".

For much of the painting's life, there had been question marks over whether it was a Titian or not, and it was gratifying, Mr Gimpel said, that his grandfather's belief in the painting had been vindicated.