To the wider public Titian himself is not as widely known nor does he seem quite as accessible as some of the other painters who have fetched high prices in the saleroom in recent years. He is not a Matisse or a Van Gogh. And his mythological subject matter, no matter how sensual its treatment may seem, can look somewhat remote from us.

However, it is difficult to overestimate the importance of these two Titians - not least because it is extremely unusual for any of his works to become available for acquisition. Picasso made 10,000 paintings during his lifetime. Titian, though he lived well past his 80th year, has left a legacy of only a few hundred, and most of these are locked into the world's great national collections.

But why is this painting so great? It belongs to a group of six that Titian painted for Philip II of Spain between 1556 and 1559. The master was already in his seventies when he painted them. What they offer us is drama of a particularly intense and sensual kind, and it is derived from one of the most popular and familiar of Renaissance literary source, Ovid's Metamorphoses.

The painting is executed with Titian's usual colouristic brilliance and tremendous dramatic verve. There is a sense of hurry and tumult. Diana, chaste goddess of the hunt, has been bathing in private in a pool. Suddenly Actaeon, the young huntsman, bursts in and the goddess, in spite of the best efforts of her nymphs, is exposed to view.

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 goddess is larger than her nymphs, quite deliberately so. She blushes at the huntsman. She looks murderously at him. Though a natural scene, the trees seem to lean down and to enfold the protagonists as if they were enclosed within a grotto. A lap dog barks furiously, as if aping the anger of his mistress. Actaeon himself rears back, alarmed. A stag's skull reminds us that Actaeon himself will soon die a brutal death. Titian's palette is, as ever, rich and dazzling, and he paints with a tremendously subtle forcefulness. Evidence of his feverish brush marks are everywhere about the surface of the painting - how he has dabbed and smeared and pushed and rubbed. What is more, the agitation of the surface mirrors perfectly the agitation of the scene so brilliantly depicted.

At least one of the greatest of the moderns, Lucian Freud - recently crowned the world's most expensive living artist - has testified to the impact upon his own life and artistic taste of the very paintings that are now up for sale. When asked in 2001 about Titian's work, Freud revealed that, when staying in the Scottish Borders, he had travelled almost every day to see the two Dianas in Edinburgh. “To me, these are simply the most beautiful pictures in the world,” he said.

And Freud is not alone. Not long before her death, I asked the novelist Iris Murdoch which was the greatest painting she had ever seen. Without a second's hesitation, she mentioned a Titian.

It would be folly to pass up the opportunity to strive to acquire this great masterpiece for the knockdown price of £50 million. There will be no second chance.

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