TITIAN received a commission in 1556 from Philip of Spain, who was newly King of England, for a series of six mythological scenes, or poesie, deriving from the classical texts of Ovid, Catullus, and Philostratus. Titian was at the height of his powers, and was soon the King's favourite painter.

It was announced on 1 March that the National Gallery in London and the National Gallery of Scotland had jointly purchased the Diana and Callisto for £45 million. The original asking price had been £50 million, but the current owner agreed a ten-per-cent discount on the sale.

Although the Diana and Actaeon was part of Philip's original commission, the King seems never to have received it, as the artist kept revising it until his death, in 1575, and some suggest that it is still unfinished.

In 2009, both galleries, with public and government support, had been able to buy the only other painting from the series still privately owned (Diana and Actaeon) also from the Duke of Sutherland for £50 million. In 1972, the London Gallery had successfully bought one of the other pictures in the series (The Death of Actaeon). For the first time since the 18th century, the three great masterpieces are side by side in an exhibition at the National Gallery, Metamorphosis: Titian 2012.

The successful acquisition of both Diana works, for which the National Gallery had to gain parliamentary permission to raid its reserve, further secures the continuing loan of a substantial number of Old Masters to the National Galleries of Scotland, until at least 2030.

The so called "Bridgewater loan" has been displayed in the Edinburgh galleries since 1945, when the wartime bombing of Bridgewater House in London led the then Duke to ask if they might be given safe keeping in a public gallery. These include three other Titians (The Three Ages of Man, the Venus Anadyomene, and The Holy Family with St John) that were originally owned by the Duc d'Orleans before the French Revolution, and then had passed to the 3rd Duke of Bridgewater; Nicolas Poussin's Seven Sacraments; and four Raphaels, of which the sublime Holy Family with a Palm Tree is perhaps the best loved.

"Is any painting worth one million pounds?" I recall being set this as an exam question for university entrance three years after an American dealer had paid a record £2.3 million for the Velázquez portrait of Juan de Pareja (c.1610-70). I probably wrote as pretentious an essay about aesthetics and value as any 17-year-old might. The artist himself thought so much of the portrait of his mulatto serving man that he had it displayed in the portico of the Pantheon when he completed it in March 1650.

At the time, the Earl of Radnor's sale at Christie's on 27 November 1970 caused an international flurry of interest. On 12 May 1971, it was announced that the Metropolitan Museum of Art had bought the painting from Wildenstein's. Only later did it emerge that the Met had sold modern works in the collection to fund the purchase, which led to a change in rules governing the right of galleries and museums to de-accession works in their custody.

Until recently, at least in the United States, such a prohibition has kept many collections together; but changes are afoot. In 2011, the Getty Museum sold 15 Old Masters, and Cleveland put 32 up for sale. The Pennsylvania Museum of Fine Arts sold off five. Long-term loans often have a dubious status that can be challenged, as the Auckland Castle sale of the series of Zurbarán's Jacob and His Twelve Sonsshows.

Issues about ownership and value are inextricably combined, and the acquisition of the Titians indicates that the debate has a long way to run. Seeing the two "Bridgewater" Titians at the heart of the Sainsbury Wing show, which includes works by Chris Ofili, Mark Wallinger, and Conrad Shawcross, makes the point that these works have always exerted an influence on later artists, and ought to be owned publicly.

The late Lucian Freud claimed in 2009 that these two Titians were "the greatest paintings in the world". Whether he, or the other artists who have suddenly acclaimed them (among them, Tracey Emin and Anthony Gormley), have put their money where their mouth is in helping to save the pictures for the nation is not clear. What is clear is that both National Galleries have acted proudly in the longer-term interests of the nation, even though the continued loan of the other paintings might outrun the Union of the Crown itself.

The scene in which Diana discovers Callisto's pregnancy before banishing her is written in Ovid's Metamorphoses II, 401-503, hence the title of the London exhibition that, with a new work for the Royal Ballet, is part of the Cultural Olympiad. Rubens was so impressed by the pathos of the scene when he visited Spain in 1628 that he copied it; and that full-size painting is now at Knowsley, Lancashire, owned by the Earl of Derby. Titian described his other work as "Diana at the fountain surprised by Actaeon" in a letter of 19 June 1559, and his canvas closely follows the story recounted in Metamorphoses III, 138-253.

With the two side by side, it is possible to see how cleverly Titian has mirrored the composition of each. The column in one becomes a fountain in the other, and the arching body of Diana in both looks at the luckless protagonists. The lining of Actaeon's hunting boots is only a shade less audacious than the red slippers that Callisto wears.

Both are bathing scenes, probably deliberately chosen to provoke the youthful appetites of the newly married King and his older wife, Mary I of England. His claim to the English throne lapsed at her death (May 1558), but it is fitting that their short marriage is commemorated by paintings now owned by the nation.

Titian's first acclaimed masterpiece, The Flight into Egypt of 1506-07, is on loan from the Hermitage in St Petersburg to the National Gallery (until 19 August), where it can be seen in the Sunley Room, alongside such early works as portraits of his patrons from Alnwick Castle and Ickworth, and an inspiring array of pictures by Bellini, Sebastiano del Piombo, and, of course, Giorgione.

This significant, and large, canvas, originally commissioned by an influential Venetian family, helps to suggest how the Titian was the first real master of landscape in the West. We take for granted the appeal that nature has for artists, but it was only through the increased scientific investigation of flora and fauna in the German Renaissance that landscape painting became an art in its own right.

The exemplary exhibition surrounding The Flight into Egypt from Russia, where it has been since 1768, helps us to explore how this explosion of interest occurred by bringing together the various elements that Titian drew upon, including the exacting studies made by Albrecht Dürer and his compatriots. This is less surprising than at first appears when we recall that Titian grew up in the mountains north of the Veneto close to the German frontier of the Holy Roman Empire, and that Dürer certainly visited Bellini's studio when Titian was working in it.

Vasari, who acclaimed the The Flight into Egypt as Titian's first great masterpiece, records that the young artist shared his house with Germans; and their influence seems inescapable in the wily fox surveying the mystical scene of Mary and Joseph being led across a sylvan field. All the animals in the scene, and the burst of poppies in the foreground, demonstrate how skilled Titian already was, and how ready he was to build on the achievements of others.

How good is the painting? On any consideration, it is an uneven work, and some commentators suggest that it is not by Titian at all. But if it is really the work lauded by Vasari, it is difficult to understand why Titian was so much better at painting portraits than figures. The landscape is richly evocative, but, alongside Bellini's The Death of St Peter Martyr, it is not really memorable until we see how clearly Titian has observed perspective.

Only because of its comparative size, it might be possible to miss seeing the other loan from St Petersburg, an equally early wooden panel painting of the Holy Family in a Landscape. It appears ingenuous alongside the much larger-scale composition of the Flight; so we can perhaps appreciate what Vasari may have been reporting. But, at the same time, it is worth pointing out that this little picture, too (or perhaps the virtually identical composition now in Raleigh, North Carolina), was famous in its day, and was engraved as early as 1515.

This exploratory exhibition in the Sunley Room brings together no fewer than ten early paintings by Titian, culminating in London's great Noli me tangere, which is here as a landscape painting and not as a devotional picture. With the Sainsbury Wing show as well, this makes London the unexpected centre of a Titian-fest, and for me justifies the Olympic bid, if not the cost of the Games.

"Metamorphosis: Titian 2012" (until 23 September) and "Titian: A Fresh Look at Nature" (until 19 August) are both at the National Gallery, Trafalgar Square, London WC2. Phone 020 7747 2885.

www.nationalgallery.org.uk