No oil painting? A labour of love secures BP Prize

When Dean Marsh painted a portrait of his girlfriend Giulietta Coates, he intended it to be a token of his love for her.

A romantic gesture, undoubtedly, until he publicly unveiled it at the National Portrait Gallery in front of the subject herself - and then admitted that it wasn't entirely flattering.

Last night, he was awarded the £25,000 BP Portrait Award 2005 for his oil painting, Giulietta Coates. Luckily, Ms Coates approved of it too.

Marsh, 36, described the portrait - the largest he has attempted - as a labour of love.

"Giulietta is my girlfriend and I wanted to paint her because I love her. It's the second time that I have painted her and it's the largest portrait I've ever done. I spent around four to six months, in total, painting the picture. I'm not sure about my approach to portraiture, except that I try to be honest. I don't flatter," he said. The painting features an intimate study of Giulietta's face, with lines around the eyes, staring directly at the viewer.

Sandy Nairne, director of the National Portrait Gallery in London, praised the winning work as an honest study of his subject.

"Dean Marsh's winning portrait is a direct and penetrating study, painted with great skill and finesse," he said. "It's quite a tough piece of painting. You feel that the woman is being portrayed as she comes, with every shadow and quite a lot of wrinkles but it feels very beautiful and beautifully accomplished. It's a worthy winner in a strong year for the BP Award."

Mr Nairne added that all the entries were, as always, submitted anonymously with no explanation of the subject matter. While the judges did not realise Giulietta was the artist's girlfriend, they sensed a "tremendous intimacy", he said.

Marsh, who lives in London, entered portrait competition in 1993, 1994, 1996 and 2000. In 2002 he made the shortlist, when he was commended for his portrait of Rosalind Savill, the director of the Wallace Collection, where he worked as a gallery attendant, and then won fourth prize for Man with Grey Scarf in 2003.

The artist, who did a foundation course at Ravensbourne Design College in London, once said he never wanted to go to art school for a longer period because he believed painting could not be taught. He also won a commission, at the trustees' discretion, worth £4,000.

The other short-listed works were Gregory Cumins' mixed media Richard Deacon and two oil paintings, Saul Robertson's The Universe and Conor Walton's Monkey Painting. The winning portrait is among 53 works on show at the National Portrait Gallery from tomorrow until 25 September.