The moment that every museum chief most dreads came to John Leighton late last year.

The Duke of Sutherland informed the director-general of the National Galleries of Scotland that he wished to sell “a significant part” of the world’s most spectacular private collection of Old Masters, which have been on public view in Edinburgh for 63 years and are the institution’s pride and joy.

The Bridgewater collection is unique, but the ripple effects of the proposed sale could reach every important museum in the country. “With the Bridgewater Collection, the National Galleries of Scotland is a ‘must-see’ in the Michelin Guide,” Mr Leighton said yesterday. “Without it, it is ‘worth a detour’.”

The collection boasts glittering masterpieces by Titian, Raphael, Rembrandt and Poussin, and represents an extraordinary proportion of its owner’s wealth. The Art Newspaper values the Titians at £300 million and the rest of the collection at £1 billion, compared with the £30 million that The Sunday Times Rich List estimates for the duke’s non-art assets.

Times Archive, 1933: The Bridgewater Titians

'Actaeon' is the most complete and satisfying of the paintings, as it is the one that has suffered least by time

• The conservation of pictures

• Picture: The vapour chamber used by Mr Kennedy North for the conservation of Titian's painting, Diana and Actaeon

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The collection is built around masterpieces acquired from the Duke of Orléans’s estate after the French Revolution. It might have been destroyed during the Second World War had the family not sent the paintings from its home in Bridgewater House, London, to its estate in the Scottish Borders in 1944. Soon afterwards, Bridgewater House was badly damaged by bombs.

The announcement of a £100 million campaign to save the crowning glories of the collection — Titian’s Diana & Actaeon and Diana & Callisto — for the British public will send a nervous shudder through every gallery director and art lover in the country. The Museums, Libraries and Archives Council estimates that between 1 and 2 per cent of all the artworks in Britain’s leading museums are private loans.

While there, they are effectively tax-exempt: inheritance tax and capital gains tax are deferred until the moment that the owner might choose to sell them on the open market. At the same time the galleries provide security and take responsibility for insuring the works. Great paintings such as the Titians are covered by government indemnity.

A booming art market and deepening economic gloom elsewhere will tempt many owners to cash in their art assets, but British museums and galleries are in a weak position to save these paintings for the public.

Wealthy Middle Eastern franchises of the Louvre and Guggenheim, Russian oligarchs, Indian techno-billionaires, American hedge-fund owners and the vast arts institutions they support all have huge budgets beyond the imagination of British gallery chiefs.

The National Galleries of Scotland and the National Gallery in London hope that their bid to buy the two Titians can succeed. “We are in uncharted waters,” Mr Leighton said. “The case [for raising the money] is formidable, as is the challenge. It can be done but it will take all the stars to come into line.”

Losing the paintings would render his gallery the equivalent of “the Uffizi without the Botticellis or the Louvre without the Mona Lisa”.

The combined fee to save the works would be almost five times the record for a work of art acquired by a British public collection — set when the National Gallery paid £22 million for Raphael’s Madonna of the Pinks in 2004. If the galleries are successful, they will take it in turn every five years to display the paintings.

Nicholas Penny, the director of the National Gallery in London, said that the Titians had had an “electrifying” impact on British art. “They changed the way that Constable painted landscapes and they were the things that Turner most admired. For a century the agitation to preserve great works of art in British collections from export has been animated by anxiety that Titian’s great paintings Diana & Actaeon and Diana & Callisto might be sold. Now the paintings have been offered on remarkably advantageous terms: their acquisition by both institutions would be an historic event.”

Raising £100 million in a troubled economy was not ideal but there was encouraging precedent, he said. “The National Gallery was founded when this country was in a very bad way and most of its major acquisitions have been made against, and because of, a background of economic distress.”

The death of the sixth Duke of Sutherland in 2000 appeared to have brought about such a moment of crisis. The title passed to his cousin, Francis Egerton, but the heavy demands of inheritance tax put a strain on the finances of the estate, which led to the sale from the Bridgewater Collection of Titian’s Venus Anadyomene, bought by National Galleries of Scotland in 2003 for £11.6 million.

Now the 68-year-old 7th Duke has again been moved to realise some of the wealth tied up in the collection.

“Over the years, the Bridgewater Collection has grown in value to the point where it is prudent to review the holding in terms of the balance of the family’s overall assets,” the duke’s spokesman said. “The duke would very much want these pictures [the Titians] to continue to be accessible to public view, and to achieve this is prepared to dispose of them at a price below market value and on other terms favourable to the nation. He would also wish for the rest of the coll- ection to remain on public display.”

Most valuable masterpieces In private hands

Rembrandt Self-Portrait with Beret and Turned-up Collar, Duke of Sutherland collection; estimated £50m-£75m

Velázquez Prince Baltasar Carlos with the Count-Duke of Olivares at the Royal Mews, Duke of Westminster private collection; estimated £50m-£75m

Titian Young Man with Cap and Gloves, Earl of Halifax private collection, lent to the National Gallery 1992-2005, now for sale; estimated £30m-£50m

Sources: National Gallery, artwolf.com

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