Who the lady with the determined chin is, why the most famous painter of her day never finished her portrait, and where she has been hanging for most of the last 400 years, remain mysterious – but she is now valued at up to £6m, as a previously unrecorded work by Rubens.

"It's a fascinating piece precisely because it is unfinished, so she seems to step into life out of nothingness," said Alex Bell, an expert on old master paintings at Sotheby's, where the painting will be auctioned next week. "Although the face and the hand are finished, you can see how he sketches in the ruff and details of the costume by dragging the paint quickly across the canvas – it gives you a wonderfully immediate feeling of how he worked."

Sir Peter Paul Rubens probably painted the woman in a magnificent Spanish costume around 1603, when he travelled from the Netherlands to Italy and Spain and perfected his technique. It may have been part of a commission from the duke of Mantua, which Rubens mentioned in letters, to add the most handsome women in the Spanish court to his "gallery of beauties", but the canvas was probably a sketch from life intended to be finished later. There is no evidence that the duke ever got her or any other beauties.

The only hints of what happened to the painting come from two red wax seals on the back, one proving that it was in Venice in the early 19th century at a time when many church and private collections were broken up and the other showing it was part of the Hanmer family's collection at their mansion, Bettisfield Park near Wrexham.

Bell believes Sir John Hanmer probably bought it in London in the 1840s, but how and when it left Italy remains one of its many mysteries. Several visitors to Bettisfield recorded their admiration of the wealth of pictures on his walls, but none seems to have spotted a Rubens and Bell does not think it has been exhibited in public before.Another lost Rubens set a record for an old master painting seven years ago, when the newspaper baron Lord Thomson paid £49.5m at Sotheby's for Massacre of the Innocents, which had been wrongly catalogued and kept in store for most of its history because a succession of owners found its blood spattered subject too gruesome for everyday viewing. The Spanish beauty is not predicted to create such a sensation, but "she's causing a lot of interest", Bell said.