A portrait of Mme Paul Girardot de Vermenoux sold by London Old Master dealer Derek Johns to the Karlsruhe Museum in Germany for €1.4m (£1.2m).

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Karlsruhe Museum, in the Baden- Württemberg area of Germany, paid €1.4m (£1.2m) for the portrait of Mme Paul Girardot de Vermenoux that is signed and dated 1764.

Executed in pastel on a single piece of vellum measuring 3ft 11.5in x 3ft 1.5in (1.2m x 95cm) – the largest known work by the artist in this medium – it is believed to have been painted because of a rivalry between two well-to-do women of the period.

Liotard was commissioned to paint Mme Vermenoux in 1764 as a gift to her doctor, Theodore Tronchin in Geneva.

It appears Mme Vermenoux was aware the artist had painted the fashionable writer Madame d’Epinay five years earlier – a picture also gifted to the Swiss doctor.

According to Johns: “The younger, wealthier and more mundane Mme Vermenoux commissioned her portrait to be three times bigger than that of her ‘rival’.”

The painting appears in the catalogue raisonné of Liotard by Roethlisberger & Loche (2008) and was previously in a private collection in Geneva.