Britain is facing the loss of a painting of supreme importance to its heritage, one of Britain’s most eminent arts figures said yesterday.

Sir Hugh Leggatt, the former Museums and Galleries Commissioner, called on the Prime Minister to save the Rubens masterpiece from being sold to an overseas collector for tens of millions of pounds, after the death of its British owner this month.

Sir Hugh said that The Apotheosis of King James I – which depicts the King being raised to the heavens by Justice while his symbols of earthly majesty, the crown and orb, are borne away by cherubs – was Britain’s most significant work by the 17th-century Flemish master.

It was the draft in oils for the monumental ceiling of Inigo Jones’s Banqueting House in Whitehall, which Charles I commissioned from the artist in 1629 in celebration of his father, James I. Rubens presented it to the King to give him an idea of the finished look.

Sir Hugh fears for the future of the painting after the death of the 6th Viscount Hampden, who had lent it anonymously to the National Gallery since 1981. He had already entered into negotiations to sell it, to secure the future of the family’s home, Glynde Place, an Elizabethan manor in East Sussex. His heirs declined to comment yesterday and refused to allow The Times to photograph the work.

Sir Hugh said: “This painting must never leave these shores under any circumstances. It is vital to our nation as the most important painting in the land.

“Gordon Brown should see that the funds are available. He can do it through the Chancellor of the Exchequer. The Government should get to grips with the situation. We might have to have a flaming row with the Government if they don’t come up with money. Unless they do, people from overseas will put in a huge bid for it. It must be saved.”

Valuing a painting of such historical importance was almost impossible, he said. In 2002 Rubens’s The Massacre of the Innocents was sold by Sotheby’s in London for £49.5 million, having been given an upper estimate of £6 million. Tax breaks for a public collection would bring down the cost to the nation for The Apotheosis.

Sir Peter Paul Rubens (1577-1640), was the preeminent artist of the 17th century as well as being a diplomat, linguist and scholar. He was knighted by Charles I, who had commissioned the Banqueting House ceiling while the artist was in London from 1629 until 1630, acting as an envoy for Philip IV of Spain.

It was during this visit that Rubens created the draft. After returning to Antwerp, where the canvases were painted, the works were shipped to London in 1635. They were in place the following year, the crowning glory of a building that had been erected between 1619 and 1622. They have remained there since, surviving the 1698 fire that destroyed most of the Whitehall palace.

Far from being daunted by the enormous scale of the commission, Rubens relished the challenge, writing in 1621: “I confess that I am, by natural instinct, better fitted to execute very large works than small curiosities. Everyone according to his gifts; my talent is such that no undertaking, however vast in size or diversified in subject, has ever surpassed my courage.”

In return, Charles I rewarded him with £3,000 – the equivalent of £218,000 today – and a heavy gold chain, although the artist had to wait three years to be paid. The sketch, which measures 95cm x 63cm (37½in x 25in), was acquired by the 2nd Viscount Hampden in the 18th century.

Sources have suggested that the advantageous tax deal that was available while Lord Hampden was alive may now be lost, and the price could go up.

The National Gallery is unable to fight for it, having struggled to save Raphael’s Madonna of the Pinks, which the 12th Duke of Northumber-land had agreed to sell to the J. Paul Getty Museum in Los Angeles for £35 million. The Tate is now pulling out the stops to buy the Rubens, submitting an application for a grant from the National Heritage Memorial Fund, whose resources are limited.

Stephen Deuchars, the director of Tate Britain, said: “This is a hugely important work.”

David Barrie, director of The Art Fund, Britain’s largest art charity, said: “We must do everything we can to preserve it.”

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