The owner of a Renaissance masterpiece by Italian artist Titian, on show at the National Gallery since 1992, has decided to put it up for sale.
The painting has been on loan the National Gallery since 1992
Portrait of a Young Man could fetch millions of pounds on the open market.
The National Gallery said it had been in "exhaustive discussions" with the Third Earl of Halifax for the past two years over the painting's future.
However, it said a joint purchase plan with the National Gallery of Scotland had proved unsuccessful.
"Both galleries regret that, despite the owner's patience, no arrangement has been found that the two galleries could finance," said the National Gallery.
It said the removal of the painting would be "a great loss" to the British public.
"This case highlights the difficulties faced by public institutions in funding purchases."
A spokesman for Lord Halifax said he had "very much hoped" the picture would remain on public view, preferably in one of the UK's national galleries.
"But their resources are not unlimited and this kind of commitment is not possible for them at this point."
The painting is the latest in a number of important artworks to have appeared on the open market in recent years.
The Earl of Halifax is the deputy chairman of Christie's UK
The Massacre of the Innocents by Rubens was sold at auction for £49.5m in 2002.
And last year a 1905 masterpiece by Picasso became the world's most expensive painting when it sold for £58m.
In February 2004 the National Gallery bought Raphael's The Madonna of the Pinks from the Duke of Northumberland for £22m.
But it is likely the Titian would fetch considerably more, placing it out of reach of the country's main galleries.
"The likelihood now must be that the picture would pass to an overseas buyer," said Lord Halifax's spokesman.
Titian (Tiziano Vecellio) is widely regarded as one of the most pioneering of all Renaissance artists.
Portrait of a Young Man formed part of the National Gallery's 2003 Titian show, the biggest exhibition of his work ever shown in the UK.
Painted in Venice in the second decade of 16th Century, the portrait depicts an unknown sitter standing in three-quarter profile behind a parapet.