The works, which have been on public display at the National Gallery in London and the National Galleries of Scotland for more than 200 years, have been put up for sale by their owner, The Duke of Sutherland.
If the galleries cannot match the £50 million asking price by the end of the year then private collectors will be invited to bid for Diana and Actaeon.
Diana and Castillo, its accompanying work, will be sold in three years’ time.
The National Heritage Memorial Fund yesterday pledged £10 million towards the fund in a move the National Gallery described as “a crucial endorsement”.
But in an open letter, Sir John Tusa, Chairman of London’s University of the Arts, and Nigel Carrington, rector at the university, qualified their support for the move by calling for more investment in young artists and education.
They said: “Young artists who study at institutions are often forgotten in the rush to celebrate the established and praise the gilded. As the costs of education become increasingly prohibitive, and the financial climate darkens, we risk losing many gifted students, especially those from less traditional backgrounds."
“If two magnificent Titian paintings are valued at £50million, what price countless future generations of artists, designers, journalists and actors?”
The remarks follow a high-profile show of support for the fund by artist Tracey Emin, who took a typically unequivocal stance by delivering a petition signed by a string of high-profile peers to Downing Street last week, including the likes of Damien Hirst and David Hockney.
“It will be really embarrassing if the government doesn't buy them and they are bought by some Russian oligarch and never seen again,” said Emin.
"If they don’t buy these, how many other paintings will they allow to be lost to the nation? We are building an Olympics that we can’t afford and can’t maintain afterwards. This country seems hell-bent on supporting a war which is so ugly. Why can’t we celebrate things that are really beautiful?”
Jenny Abramsky, Chair of the NHMF, said: “This is exactly what the NHMF was set up to do. We have moved quickly to show our support and hope this persuades others to dig deep to make this dream a reality.
“It was an extraordinary and challenging decision for us financially, but it’s as important as ever to protect our enduring cultural heritage – those things of outstanding quality that enrich our national life.”
The paintings are the jewels in the crown of the Duke’s Bridgewater collection of 27 Old Master paintings, and are believed to be worth in the region of £300 million.
In August, The Times reported that Culture Secretary Andy Burnham had asked the treasury to provide an emergency grant of £10 million for the cause.
John Leighton, Director-General of the National Galleries of Scotland, said: “This extremely generous and wholehearted support from the NHMF 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.”
