An Ashmolean campaign to keep Edouard Manet’s Portrait of Mademoiselle Claus in Oxford has been given £5.9m Lottery funding.

The 1868 portrait has been owned by the family of artist John Singer Sargent since 1884.

The family had 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 Since launching the campaign in February, the Ashmolean has also been awarded a significant grant from The Art Fund of £750,000, along with £200,000 of gifts from individuals and The Friends of the Ashmolean.

The portrait is a preparatory study for Le Balcon (1868–9) now in the Musée d’Orsay and one of the key images of the Impressionist movement. Having been exhibited only once since it was painted, it is on display in the Ashmolean’s Impressionist Gallery.

Carole Souter, chief executive of the Heritage Lottery Fund, said: “This study of a young woman in repose is extraordinary: luminous, beautiful, a real masterclass in brushstroke technique.

“HLF is pleased to play a significant role helping the Ashmolean secure Manet’s Portrait of Mademoiselle Claus, particularly as it is unlikely that a work of this calibre will become available again at such a competitive price.”

The Ashmolean has until August 7 to raise the remaining £980,000 before the export bar ends.