The campaign to keep Titian's masterpiece Diana and Actaeon on public display has been given a £10m boost.
The National Heritage Memorial Fund (NHMF) announced it would donate the sum to The National Galleries of Scotland and the National Gallery.
The two institutions are hoping to raise £50m to jointly buy the painting before a deadline of 31 December.
Jenny Abramsky, chair of the NHMF, said it had been a "extraordinary and challenging" financial decision.
But she added it was as "important as ever" to protect the UK's cultural heritage.
Diana and Actaeon has been on public display at the National Galleries of Scotland and London for more than 200 years but it is now being sold by its private owner, the Duke of Sutherland.
If the galleries raise the funds to buy Diana and Actaeon, then a second Titian painting - Diana and Callisto - will be offered for sale in four years.
If they fail to raise the required £50m then Diana and Actaeon will be sold to a private buyer.
Tracey Emin, Lucien Freud, Damien Hirst and David Hockney are among the artists campaigning to keep the works in the UK.
Ms Abramsky said: "We have moved quickly to show our support and hope this persuades others to dig deep to make this dream a reality."
The two Titians were created as part of a cycle of works for King Philip II of Spain from 1556 to 1559 and form part of the Bridgewater collection, which has been on loan to the National Galleries of Scotland since 1945.
The collection also includes three paintings by Raphael, a Rembrandt, a Van Dyck, and a Tintoretto.
Tracey Emin is one of many artists supporting the campaign
Should the two galleries manage to raise the necessary funds, the entire collection will remain on long-term loan to the Scottish gallery.
John Leighton, director-general, National Galleries of Scotland said: "This extremely generous and wholehearted support represents a major boost for the campaign to acquire this superlative masterpiece for the national collections.
"In supporting this acquisition the Fund has also recognised fully the vital importance of keeping the Bridgewater collection on view in Scotland and available for the inspiration and enjoyment of the present and future generations."
Nicholas Penny, director of the National Gallery London, said the donation represented a "crucial endorsement" of the value of acquiring the Titian.