Portrait of a Young Woman, attributed to Rubens, which could be kept in the UK following a temporary export bar. Photograph: PA

An enigmatic Spanish beauty, unsigned but alleged to be a lost Rubens, has been temporarily barred from export, to allow a UK museum or gallery a chance to raise £1m to keep her in Britain.

The painting had an estimate of up to £6m when offered for sale at Sotheby's 13 months ago by the anonymous owner who bought it 25 years ago. But it failed to sell owing to the uncertainty over the work – which was never finished.

Little is known of the painting or its history, neither the identity of its subject matter – a diminutive woman with a direct stare, sumptuous jewellery and costume – nor where the work has been during the past 400 years. The attribution to Rubens is based on the style, particularly the virtuoso dragging of the paint across the canvas to create the ruffles.

The work dates from around 1603, and has two red wax seals on the back, one proving it was in Venice in the early 19th century, a time of political turmoil when many church and private collections were broken up, and the other showing it was in a British collection by the 1840s, among the paintings bought by Sir John Hanmer for his Bettisfield Park mansion in Wrexham. Although many visitors recorded their admiration of the art on his walls, nobody seemed to have singled the painting out, still less as a possible Rubens. Experts at Sotheby's believe when the work went on show before the sale in December 2009, it was the first time it had ever been publicly exhibited.

The committee which advises the government on which works of art should have their export licences delayed – they must be judged of high quality and with a significant British connection to qualify – described the painting as outstanding, both aesthetically and for the study of early 17th-century portraiture. Lord Inglewood, chair of the committee, said the work was "a striking portrait of a very real, although unidentified, woman". The export bar is until March, but may be extended to May if a museum shows that it is serious about raising funds to buy the painting. The £1m price tag would be a bargain for a genuine Rubens, but very steep for an anonymous portrait by a painter following his style.

Rubens had travelled from the Netherlands to Italy and Spain in the early 17th century to study southern painting techniques and perfect his own. The unsigned work could have been part of a lucrative commission from the duke of Mantua which he mentioned in letters, for portraits of aristocratic Spanish women to add to his "gallery of beauties" – though there is no evidence that the duke ever actually got her or any other beauties from Rubens.