The five-foot high painting hung for several years in Mansion House in Doncaster before a disagreement between Doncaster council and Hutchinson resulted in the artist taking it back, later selling it to Ms Brown. It is expected to fetch up to £8,000.

The Queen Mother sat five times for Hutchinson, who was the illegitimate child of a Scottish nobleman and his 15-year-old Anglo-Indian servant.

The artist asked his royal subject to wear the Koh-i-Noor diamond, which was brought from the Tower of London by a brigade of soldiers, causing Her Majesty to observe that she had only three policemen for her own protection.

“The Queen Mother (in contrast to her daughter) was not encumbered by her role,” Hutchinson later recalled.

“She was difficult to paint because she moved around and was easily distracted. She was sweet and charming and  had a great sense of humour and insisted that I keep coming back to finish the painting to the evident impatience of the members of her household.”

He described her face as “a beautiful moon with craters”.

Link with Countess Mountbatten

Prince Philip was the first royal Hutchinson painted, possibly due to his link with Countess Mountbatten, his aunt, who was patron of the West Bengal orphanage where the artist was raised.

She recognised his talent and offered to sit for him when he was just 18, prompting Hutchinson to later note: “She made me feel like a great painter and not like a child just out of school.”

Hutchinson, who died in 2010 aged 79, described the late Duke as “difficult at first because he was very reserved”, but added: “It worked out well in the end.”