Cash in the attic: The painting by John Everett Millais, a portrait of Effie Gray who later became his wife, was discovered in a woman's loft and is estimated to be worth £50,000

A painting worth £50,000 has been found behind an old mattress in an attic where it had lain collecting dust for decades.

The 45-year-old woman owner had been given the art work for her ninth birthday but had kept it in her loft for years and forgot about it until she put her house up for sale.

An eagle-eyed auctioneer who was valuing her belongings spotted the oil painting and was stunned when he realised it was by 19th century artist John Everett Millais.

As well as being worth thousands, the dusty painting has also shed light on a 150-year-old scandal of a love triangle involving Pre-Raphaelite artist Millais.

The portrait is of a woman called Effie Gray who was married to Millais' mentor and art critic John Ruskin at the time.

Millais, whose work includes the famous Ophelia painting, met and fell in love with Effie in 1853 while he used her for another painting, the Order of Release. Effie was in a loveless marriage with Ruskin and left him to marry Millais three years later. They went on to have eight children together.

The portrait of Effie was bought in 1972 by the unnamed vendor's mother who gave it to her daughter as a birthday gift.

Auctioneer Duncan Chilcott had been asked to inspect an antique table in the attic of the 45-year-old woman's detached home in Somerset when he stumbled upon it.

He said: "The lady is downsizing and needs to sell some of her belongings. Up in the attic she wanted me to look at a table. On the other side of the room behind some old mattresses I saw the painting leaning against the wall.

"It was covered in thick dust and I was astonished when I blew it off and saw what was beneath.

"The lady selling it said it was purchased for her by her mother as a ninth birthday present. But judging by the layer of dust on it, it had been in the attic for a very long time, during which it has been lost to the art world."

The painting, which has Millais' signature on the back, shows Effie Gray in a "passionate" and thoughtful pose while she was aged in her mid 20s.

Elizabeth Chilcott, who runs auctioneers Chilcotts in Tiverton, Devon, with her husband, said: "There's a great deal of passion in the picture. We don't know for sure as to when exactly it was painted.

"What is interesting about it is that her wedding ring finger is hidden by her other finger and a shadow so it is intriguing as to whether she was wearing it or not at the time."

The 14ins x 11ins painting was auctioned at Christies in 1961 and bought by Bond Street dealer Jeremy Maas. He later sold it and it was eventually bought by the vendor's mother at an art gallery in 1972.

Mrs Chilcott said: "It was given to her daughter for her birthday, something that she could appreciate for a long time. The vendor knew what it was but it was just something she has lived with over the last 30 years and didn't think about it.

"When we told her its value she wasn't leaping up and down but she was surprised by it. I guess it is cash in the attic for her."

Mrs Chilcott said there had been a great deal of interest in the little-known painting, adding: "The prospect of bringing it back in to the pubic domain is an exciting one."

The painting has an estimate of £50,000 pounds and the auction is on December 9.