It is a masterpiece of supreme importance to Britain's heritage, and the Tate has only a few weeks to save it for the country.

The Flemish master Peter Paul Rubens created the oil painting for Charles I in the 1620s to show his ideas for the ceiling at the Banqueting House at Whitehall. But The Apotheosis of James I: Multiple Sketch for the Banqueting House Ceiling is in danger of being sold overseas and yesterday the Tate asked for the public's help to raise the £6million it needs to buy it.

The historian David Starkey gave his support to the campaign, saying that it would be a “fundamental betrayal” of Britain's heritage if such a work were allowed to be sold. “It is unthinkable that this leaves the country. Mostly it doesn't matter where a Rubens or Turner is but when you have these associations of history, place and biography, then it really does matter.

“This work is of the utmost significance to British history.”

The sketch gives a crucial insight into Rubens's earliest ideas for seven of the nine finished ceiling canvases. The Banqueting House and his ceiling are all that remain of the original Whitehall Palace — once the largest palace of Northern Europe — which was destroyed by fire in 1698.

The painting, which is being sold by the family of the 6th Visount Hampden, who died in January, has been valued at £11.5million but it could fetch more. Another Rubens painting, The Massacre of the Innocents, was sold in 2002 by Sotheby's for £49.5million, well above the presale estimate of £6 million.

Although tax breaks for a public collection have brought down the cost of the work to £6million, the special offer expires on July 31.

The Tate, with support from the leading art charity The Art Fund, said that it had already secured more than £1.56million, including a grant of £500,000 from the Art Fund and £300,000 from Tate members.

The public appeal follows calls in The Times in January by Sir Hugh Leggatt, the former Museums and Galleries Commissioner, for the Prime Minister to intervene and save the masterpiece.

Rubens (1577-1640) was a diplomat and linguist as well as an artist. From 1629 to 1630 he was an envoy to London for King Philip IV of Spain and was even knighted by Charles I.

In between peace negotiations between England and Spain, he discussed an art commission to celebrate the glorification of Charles I's father, James I, and the union of Scotland and England.

Rubens depicted James I being raised to the heavens by Justice, while his symbols of earthly majesty, the crown and orb, are borne away by cherubs.

The ceiling canvasses are massive. Two of them measure 13x3 metres (approximately 40 x 10ft). Far from being daunted by the scale, Rubens relished the challenge. “Everyone according to his gifts,” he wrote. “My talent is such that no undertaking, however vast in size or diversified in subject, has ever surpassed my courage.”

The painting will be on display at Tate Britain until July 31. Anyone wanting to donate money can do so at the Art Fund's website which can also be accessed via www.tate.org.uk/rubens

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