In a week when Damien Hirst reinforced his reputation as the king of the saleroom with a £111 million auction at Sotheby’s, Julie Staniforth thought that she too may be sitting on a fortune.

She is the owner of an original Hirst executed by the artist when he was a teenager in love with her sister.

The oil portrait of Ms Staniforth’s late cat Mindy may be unique – Hirst’s figurative paintings are usually substantially the work of artists in his studio – but it is also worthless, according to valuers at Sotheby’s.

An expert told her that there was no demand for works that bear no relation to the artist’s usual installations of animals suspended in formaldehyde and cabinets filled with pharmaceuticals.

Ms Staniforth, 51, a carer from Alwoodley in Leeds, acquired the painting when the artist was going out with her sister, Joanne, some 26 years ago.

“My younger sister Joanne, who is now a mobile hairdresser, was dating Damien at the time,” she says. “They were childhood sweethearts. They were at the same school.”

Hirst was the son of a motor mechanic who left home before his rebellious son was in his teens. His mother was a woman of sense and exacting standards, although she had trouble controlling her wayward son.

But Mrs Staniforth remembers the 17-year-old Damien as a “lovely boy – very lively, full of fun, full of jokes, a really nice person”.

He would see her sister every night and then put her safely on the 10 o’clock bus home. He was also one of the best in his art class, so she gave him a photo of Mindy perched on the arm of a chair and asked him to copy it. He took it home and a few days later he came back with this lovely little oil picture,” she says.

“He didn’t want any money for it – he wouldn’t even have thought of that then – and I hung the picture on my bedroom wall.”

Joanne Staniforth, 43, remembers Hirst fondly even though he broke her heart. “I was gutted when he finished with me three weeks before my 18th birthday but we stayed friends and he still came to my party,” she says. “We went out for quite a while, over a year. He was great. He was very funny, very individual and good at playing jokes. He stood out from the crowd and it was obvious at the time he was going to do well. He was the star pupil in the art class.

“I didn’t have any more contact with him after that but I’m really pleased for him. I think his success is fantastic. I’m happy to have been a little part of his life.” The painting is a perfectly competent picture of a cat. Mindy, a grey tabby, looks plump and happy with her pretty white bib and pair of newly washed socks. It is questionable whether she would have looked so relaxed if she had known that Hirst would soon have preferred to pickle her than to paint her.

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