The Rubens masterpiece The Apotheosis of James I, described as of supreme importance to Britain’s heritage, has been saved for the nation after a last-minute donation to prevent it from being sold abroad.

The Times revealed in January how the oil, a preparatory sketch for the canvases that Rubens painted for the ceiling of the Banqueting House in Whitehall, was at risk of being lost to foreign owners. The Tate announced yesterday that it had raised £6 million, helped by public donations, to acquire the work, days before it would have gone on the open market.

Stephen Deuchar, the director of Tate Britain, paid tribute to this newspaper for raising awareness of the picture’s plight. The public responded to appeals by Sir Hugh Leggatt, the former Museums and Galleries Commissioner, for the nation to save “the most important painting in the land”. Yesterday Sir Hugh said that the news was wonderful, adding: “It would have been the greatest tragedy if we hadn’t saved it.”

The Apotheosis was sold by the family of the 6th Viscount Hampden, who died in January. His heirs agreed to waive £300,000 from the asking price of £6 million. Tax breaks for a public collection brought down the cost of the work, which was valued at £11.5 million, although it would have fetched far more if it had been put on the open market.

The 17th-century Flemish artist painted his masterpiece for his monumental scheme of ceiling paintings in the Banqueting House — one of the crowning glories of the architect Inigo Jones. The Banqueting House and his ceiling are all that remain of the original Whitehall Palace — one of the largest palaces of northern Europe — which was destroyed by fire in 1698.

Mr Deuchar said: “I am simply thrilled. This is the first Rubens work to enter Tate’s collection and is a unique treasure in the history of British art. By acquiring this painting we can begin to represent the magnitude of Rubens’s importance in British culture.”

Valuing a painting of such historical importance was almost impossible. Another Rubens painting, The Massacre of the Innocents, was sold in 2002 by Sotheby’s for £49.5 million.

Rubens created The Apotheosis for Charles I in the 1620s to give the King a taste of how magnificent the finished ceiling would look. Today, the oil painting gives an insight into the artist’s earliest ideas for seven of the nine finished ceiling canvases.

Rubens was a diplomat and linguist as well as an artist. In 1629-30 he was an envoy to London for King Philip IV of Spain and was 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. He 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 canvases are massive. Two of them measure 13m by 3m (40ft by 10ft). Rubens relished the challenge, saying: “My talent is such that no undertaking, however vast in size or diversified in subject, has ever surpassed my courage.”

The Tate’s purchase was made possible by the Art Fund, the leading independent art charity, which granted £500,000 and later an additional £100,000 to help to bridge the final gap. The biggest donor was the National Heritage Memorial Fund, the nation’s fund of “last resort”, which granted £2 million in July and then pulled out all the stops to find a further £221,000 on the final day. The historian David Starkey said: “The loss of the sketch would have been a fundamental betrayal of our national heritage and I am delighted that it will remain on public display in London.”

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