The Tate Britain museum in London has launched a campaign to raise the remainder of the £6 million ($12.1 million Cdn) it needs to purchase a Rubens oil sketch.

Officials this week unveiled the sketch, which dates from 1629 to 1630 and is entitled The Apotheosis of King James I. It was the model for the ceiling of the Banqueting House of the Palace of Whitehall — the  main London residence of English monarchs from 1530 to 1698, when it was all destroyed by a fire, except for the Banqueting House.

According to the Tate's website, the Rubens drawing has national import because "no other sketch for the overall composition of the ceiling exists, making this a unique treasure in the history of British art."

Price settled after viscount's death

It is currently on loan by the family of Viscount Hampden, which had owned it for 200 years. The family settled on the £6 million price, much less than its estimated value of £11.5 million ($23.3 million Cdn).

It took months before a price was agreed upon due to the historical significance of the sketch. The family negotiated the price after the death of the 6th Viscount Hampden on Jan. 4.

"By acquiring this painting, we can begin to represent the magnitude of Rubens' importance in Britain," Tate Britain director Stephen Deuchar told The Art Newspaper in a web-exclusive article.

The oil sketch is thought to have been created for the approval of Charles I.

The museum has already collected £1.5 million ($3 million Cdn) from its members and the Art Fund — an independent charity dedicated to saving art for the public to enjoy.

The Tate is hoping to get the rest of the money from the British National Heritage Memorial Fund as well as other donors.

The sketch hung at the Sussex home of Viscount Hampden until 1981, when it was lent to the National Gallery.

The National Gallery decided to hand it over to the Tate last summer after it faced a choice of either buying the Rubens or a set of five highly valuable works by French classical painter Nicolas Poussin.

Peter Paul Rubens was a 17th century Flemish painter whose sensual and vivid baroque style is imbued in the many altarpieces, portraits and historical paintings he created.

His paintings have sold for more than $70 million in recent auctions.