Many things have been called national treasures, but Rubens' The Apotheosis of James I and Other Studies is the genuine article. Done in oils, the study for the ceiling of the Banqueting House in Whitehall is three feet high, valued at £11 million and depicts with swirling drama the ascent of the King to Heaven.

The work, long owned by a British family, is now on the market and the highest bidder is likely to come from overseas. Unless an appeal, launched by the Tate and supported today by the historian David Starkey, can raise enough money to buy it, it will be lost to Britain, which would be a shame.

For, although Rubens was a Flemish painter, he and the painting are part of British history. Rubens was a diplomat as well as an artist, and he came to Britain in 1630 to persuade tetchy kings to sign a peace treaty. It was while on this visit that Charles I asked him to decorate the ceiling of the Banqueting House, which he did, in nine huge panels. They are still a resplendent part of art in Britain. And fortunately, they will remain so because they are so irrevocably incorporated into the palace that the country cannot sell them off, no matter how poor it gets. The sketch, as an early phase of the art on the ceiling, can be shipped elsewhere. However, it should not be.

The argument for keeping The Apotheosis in Britain is not a strict economic one. The market, in most cases, should be left to do its work. But art and culture can have a value beyond price. Rubens' sketch is not just beautiful, it is part of the nation's heritage; and the Tate, and David Starkey, are right to campaign to raise the money for it to stay.