Diana and Actaeon was painted in the 16th century
A grant of £1m has kick-started a campaign to keep Titian masterpiece Diana and Actaeon on public display.
The donation is the largest towards saving an individual work in the Art Fund charity's 105-year history.
Two galleries are hoping to raise £50m to jointly buy the painting before a deadline of 31 December.
John Leighton, director of the National Galleries of Scotland, called the grant "a very generous donation" and said it was "a huge boost" to its campaign.
The director of London's National Gallery, Nicholas Penny, said he was "deeply grateful" to the Art Fund for its grant.
The Art Fund's director, David Barrie, said: "Today we have put our money on the table.
"Now others must come together if this extraordinarily important painting is to be kept where it belongs," he added.
If the galleries successfully raise the funds for Diana and Actaeon, then a second Titian, Diana and Callisto, will be offered for sale.
The associated artworks are part of the Bridgewater collection, owned by the Duke of Sutherland and on loan to the National Galleries of Scotland since 1945.
The Duke decided to sell the two paintings to raise £100m.
If the galleries successfully come up with the funds, the Bridgewater collection will remain on long-term loan to the Scottish gallery and available for public view.
The collection also features works by Raphael, Rembrandt and Tintoretto.
Mr Leighton said earlier this year that losing Diana and Actaeon "would be like the Mona Lisa being taken out of the Louvre".