A picture of artist Sir Herbert Gunn's daughters, painted secretly in a park after he was banned from seeing them by his ex-wife, is to be auctioned.

Sir Herbert came to a tacit agreement with their nanny to paint the children while they were playing in Regents park

Sir Herbert painted the portrait group after meeting the girls by chance at London Zoo in 1929 after he had been barred from seeing them for three years by former wife Gwen Hillman.

In 1926, Hillman ran off with one of Sir Herbert's sitters, Sir Arthur Whinney, taking Pauline, Elizabeth and Diana with her and insisting their name be changed to Whinney, leading to a bitter divorce.

After bumping into his daughters, Sir Herbert came to a tacit agreement with their nanny to paint the children while they were playing in Regents park.

Chloe Miller, his daughter from a second marriage, said: "In 1929 he was working on a mural for Turner Layton, the composer and vocalist, and went to the Zoo to make some animal studies.

"There by the lion house, he chanced to meet all three children with the nurse. The painting was produced at four or five subsequent meetings in Regents Park.

"The nurse knew Daddy but just kept knitting. She did not look up at this charming chap who happened to be painting and chatting to her charges."

Soon afterwards the girl were stopped from going to Regents Park and did not see their father again until they were adults. The painting was never fully finished.

Now the canvas, titled The Design for A Portrait Group, is expected to fetch a six figure sum when it is auctioned at the British Antique Dealers Association Antiques and Fine Arts Fair which takes place in London between March 25 and March 31.

Sir Herbert, who is best remembered for having painted the official portrait of The Queen in her 1953 coronation robes, kept the picture in his studio until his death in 1964. The oil painting has remained in the family until the present day.

He was born in Glasgow and studied at the Glasgow School of Art before attending the Academie Julian in Paris. He remained in Paris until 1914 and gained a national reputation as a society portrait painter during the 1920s. Sir Herbert was also President of The Society of Portrait Painters and a member of the Royal Academy. He was knighted in 1963 for services to art.