The Ashmolean Museum in Oxford has launched a campaign to buy a painting by the French Impressionist Edouard Manet and stop it leaving the UK.

Portrait of Mademoiselle Claus, painted in 1868, has been owned by the family of the artist John Singer Sargent since 1884 and has only been exhibited once.

The family have agreed a sale with an unnamed foreign buyer for £28.35m.

But the UK government has imposed an export bar and will let the museum buy it for £7.8m if the funds can be found.

The Ashmolean would be able to purchase it for the lower amount because the government would waive the £20.5m tax that would be required with a private sale.