The Tate's Rubens campaign underlines a Birmingham bargain, writes Terry Grimley.

A campaign to save an important painting by Rubens for the nation appears to be turning into a cliffhanger.

Tate Britain has just two months left to raise £6 million to buy a sketch for the Banqueting House ceiling in Whitehall. The decorative scheme for the building designed by Inigo Jones was commissioned by Charles I in 1629 and was designed to reflect the glory of the Stuart dynasty.

The Apotheosis of James I has been owned by the same English family for 200 years and was previously on loan to the National Gallery. It is the original sketch for the overall ceiling and is believed to have been the only one painted in London. Others, and the ceiling panels themselves, were painted in Antwerp.

About 16 of the Antwerp sketches survive and are scattered among collections from Amsterdam to St Petersburg - and Birmingham. In 1985 Birmingham Museum & Art Galllery bought James I uniting the Kingdoms of Scotland and England for £650,700 - which now seems quite a bargain.

This painting had been on loan in Birmingham for years and was offered at a specially discounted price. But the £6 million for the Tate painting is also a cut-price deal - its market value is estimated at £11.5 million. So far, according to the Tate's website, just £1.5 million has been raised, which must question whether the target can be reached. However, it is often private wheeling-and-dealing which eventually clinches public appeals. In fact, a press conference as been called for Thursday.

The cantankerous art critic Brian Sewell has criticised the Tate's campaign, arguing that Rubens' painting belongs in the National Gallery alongside more of his own work and that of his European peers - which is fair enough, except that the National faced the loss of a number of long-term loans last year and was too overstretched to mount a campaign.

Ironically, something similar happened in Birmingham when it acquired its Rubens. As well as facing the possible loss of that painting it also lost another long-term loan, a beautiful early painting by Gainsborough, which it was powerless to stop going to auction.

It went for less than £100,000 - a rare case of a painting fetching less than its estimate. Who bought it? Tate Britain, which presumably knows a mint Gainsborough when it sees one.

* The Apotheosis of James I is on view at Tate Britain until June 20. For more information about the appeal, visit www.artfund.org/savetherubens.