Prejudice, reverse snobbery or an aversion to cutting tax deals - why did Britain lose its Gulbenkian collection?

Even the most casual visitor to the National Gallery after the end of the Second World War could not have failed to be moved by the sight that lay in the first exhibition hall above the stairs.

Oil paintings by Rubens and Rembrandt hung beside the works of Monet, Renoir, Fragonard, Degas and Gainsborough. Beside the paintings, a dazzling wealth of sculptures, Islamic and Armenian art, ancient Greek coins and Egyptian artefacts stood on plinths and in glass cabinets.

The collection was on loan from Calouste Gulbenkian, an oil magnate. And in an expression of his apparently limitless generosity, it was on the verge of being given to the gallery.

Gulbenkian had agreed to bequeath it to the National, along with a dedicated annexe to house it - all funded by his fabulous fortune.

His wealth would elevate the gallery to a position of immeasurable power in a post-war era when priceless pieces were being hastily sold and the British art community was struggling to keep great works on these shores.

But after years of negotiation in which the deal was tantalisingly close to realisation, it slipped through the establishment's fingers.

The circumstances around the botched agreement have remained a mystery for decades. But recently published gallery records and government papers have revealed that it was the National's reluctance and not Gulbenkian's that led to the missed opportunity.

In 1950, the Armenian-born businessman instructed the gallery to send the collection to Washington, which led to its permanent removal from Britain and its eventual settlement in Lisbon as the Gulbenkian Foundation, where it is now one of the largest foundations in the world with an endowment of $2.6bn (£1.5bn) and an annual budget of $102m. The latest revelations have led some to argue that the suspicious attitudes of the government and gallery officials led to the greatest gaffe in the history of art patronage.

A British national since 1902, Gulbenkian was classed as a "technical enemy" in 1942 because he continued to live in Vichy France after the German invasion. Though the classification was revoked the next year, it was not fully forgotten by either the government or Gulbenkian. An internal government memorandum in March 1944 called him a "slippery benefactor" but in a sign that it was willing to make concessions to acquire a collection and endowment of such worth, the memo added: "the prize is so great that it is worth making a gesture to keep him sweet even at some expense."

But the deal was further soured by the retirement in 1945 of the director of the gallery, Kenneth Clark, a close friend of Gulbenkian.

His successor, Philip Hendy, was vehemently opposed to Gulbenkian's vision of a dedicated annexe and did not hide the fact from the billionaire.

Gulbenkian had earned his wealth by helping to set up the Turkish Oil Company, 5 per cent of whose stock went to him as the "negotiator", earning him the moniker "Five Percent".

Such was the family's wealth that he was interested in "only the best" artwork and his son, Nubar, once remarked of a new car that "it turns on a sixpence, whatever that may be".

Summing up Gulbenkian's artistic tastes, his son-in-law, Kevork Essayan said he was only interested in "big names" to augment the worth of his eclectic collection.

"As his taste improved, he became interested in great masterpieces only. When he was speaking about art, he would say with no presumption of false humility - for he was fundamentally a simple man - 'only the best is good enough for me'," Mr Essayan said.

Some have suggested that it is precisely the flamboyant nature of his collection that inspired artistic snobbery within the art world and perhaps in Mr Hendy, who was believed to be loath to build an annex to house a collection which included a bric-a-brac of furniture and other art forms as well as paintings.

Jonathan Conlin, an art commentator for The Times Literary Supplement, has speculated that Mr Hendy's fierce opposition may even have been linked directly to a question of "taste". In a TLS article, he wrote: "It would probably have been more accurate to conclude that Hendy resented the idea of having to put on show Gulbenkian's works, which even Clark had been willing privately to concede betrayed a 'rich man's taste'."

But Brian Sewell, the art critic, felt that more sinister motives lay behind Mr Hendy's obstructiveness.

"Hendy had a violent dislike of Gulbenkian because he was Armenian and Hendy was a Jewish socialist. [Philip's daughter later said this was not an accurate description of him.] He was absolutely bloody-minded and almost set out to offend Gulbenkian," he said. "Hendy should not have been director - he was the worst director since the war and was known to be a difficult man."

Mr Sewell added that suspicion over Gulbenkian's friendly gesture was tied to the socialist politics of the time as well as xenophobic sentiments.

"There was a good deal of embarrassment at the time and people were covering things up and denigrating Gulbenkian's collection. There was a good deal of class prejudice the other way round and he got caught up in it," he said.

Sir Oliver Millar, the former surveyor of the Queen's pictures, saw Gulbenkian's collection after the war. He said: "It lacked common sense to let such a collection go. There was a curious Puritanism and lack of adventure at the time."

Anna Somers Cocks, the group editor for The Art Newspaper, felt that as a hard-nosed businessman, Gulbenkian would not have let his temporary branding as the "enemy" get in the way of a business arrangement. Gulbenkian's offer had a fiscal aspect which would have greatly benefited him as he was spurred by the desire to avoid British death duties.

"He wanted a series of tax exemptions in return for the collection and it was completely inconceivable in Britain for that to happen. He was looking after number one and we need to examine his tax situation and how that related to the bequeathing of his collection," she said. Ms Somers Cocks said that British museums have never been accommodating of "conditions" attached by benefactors and the leverage they provide.

Building an annex for a separate collection can be divisive because it does not always blend in with the rest of the works displayed in the gallery and this may have proved the case had Gulbenkian succeeded in establishing himself in Britain.