Leighton

The auction house Bonhams says the painting, made by British artist Clare Leighton, is "thought to be the only oil portrait that Gandhi actually sat for".

"This is a painting of unique historic and cultural significance. It would be great if it could be seen and appreciated more widely, whether in India or elsewhere," Caspar Leighton, a great nephew of the artist, told the BBC.

According to Bonhams, Clare Leighton "was one of the very few artists admitted to his office and was given the opportunity to sit with on multiple occasions to sketch and paint his likeness".

In November 1931, Leighton showcased her portraits of Gandhi at an exhibition at the Albany Galleries in London.

There doesn't seem to be any public record of the oil portrait being displayed elsewhere until 1978, when the Boston Public Library organised an exhibition of Leighton's works.