A Rubens masterpiece commissioned by Charles I for the ceiling of the Banqueting House in Whitehall will be lost to the nation unless £6 million can be raised in the next four months.

The Tate will today make a last-ditch appeal for the public’s help to acquire the country’s most significant work by the 17th-century Flemish master, painted at the height of his powers. If it fails, The Apotheosis of King James I will be sold overseas.

The oil painting was created for the monumental ceiling in the Banqueting House, one of the preeminent 17th-century buildings designed by the British architect Inigo Jones. It was commissioned by Charles I in 1629 to celebrate the glorification of his father, James I, and depicts the monarch being raised to the heavens by Justice. It was the painting that Rubens presented to the King to give him an idea of how magnificent the finished ceiling would look.

Rubens (1577-1640), also a diplomat, linguist and scholar, was knighted by Charles I and received the commission while he was in London from 1629 until 1630, acting as an envoy for Philip IV of Spain.

Predicting what a painting of such historical importance would fetch on the open market is almost impossible. In 2002 Rubens’s The Massacre of the Innocentswas sold by Sotheby’s in London for £49.5 million. As the painting is being sold by the family of the 6th Viscount Hampden after his death in January, tax breaks for a public collection have brought down its cost to the nation from a conservative valuation of £11.5 million.

The painting, which measures 95 x 63cm (37 x 25in), was acquired by the 2nd Viscount Hampden in the 18th century. He tried to sell it in 1799, but after consigning it to Christie’s as lot 102, it failed to sell. Christie’s is now negotiating a private treaty sale on behalf of his descendants, who have agreed to lend the painting to Tate Britain to help the fundraising appeal.

The gallery has secured £1.5 million, which includes £500,000 from The Art Fund, the art charity, and £300,000 from Tate members.

Stephen Deuchar, the director of Tate Britain, said: “If the painting is unable to come into a public collection an auction is the obvious next step . . . We must secure the work before it gets into the arena of the market-place.”

David Barrie, the director of The Art Fund, said: “It is hard to think of any work of art that better deserves to be saved than this.”

In January Sir Hugh Leggatt, the former Museums and Galleries Commissioner, appealed for the Prime Minister to save the painting. He reiterated those calls yesterday.

Martin Bailey of The Art Newspaper said that the Tate’s chances of buying the Rubens will now be dependent on getting a grant from the NHMF.

The Apotheosis of James I has a temporary display at Tate Britain from today until July 31, the deadline for the money to be raised. The Art Fund has a created a website for the campaign: www.artfund.org/savetherubens . Donations can be made from the website, which can also be accessed via Tate Online: www.tate.org.uk/rubens

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