Lurid insect imagery secures prize for The Shape of Her, just ahead of Alastair Campbell, who was disqualified for wanting to win

With one killer sentence using the image of a butterfly collector – "like a lepidopterist mounting a tough-skinned insect with a too blunt pin he screwed himself into her" – the novelist Rowan Somerville demolished all comers and secured this year's coveted Literary Review Bad Sex award.

The Shape of Her is Somerville's second novel. He graciously accepted the honour, presented by film director and food critic Michael Winner, saying: "There is nothing more English than bad sex, so on behalf of the entire nation I would like to thank you."

The judges were also impressed by his nature notes, such as the pubic hair "like desert vegetation following an underground stream", and the passage: "He unbuttoned the front of her shirt and pulled it to the side so that her breast was uncovered, her nipple poking out, upturned like the nose of the loveliest nocturnal animal, sniffing the night. He took it between his lips and sucked the salt from her."

Somerville narrowly defeated Alastair Campbell, nominated not for his political diaries but his novel Maya. The judges felt his naked enthusiasm for winning disqualified him.

In this week's Observer, Campbell explained: "People have wondered if I am bluffing when I say I want to win. I do know that on Monday night they will have a very good laugh if they read certain passages out aloud. There is a bit where my central character describes a pair of breasts as 'perfect desirable objects' and they may well think that is a wanky line. But Steve is an unreliable narrator and that is the way he sees it. If some people get aroused by reading the sex scene, then fine, but these lines are mainly about the significance of this moment for the character."

The award was established in 1993 by the late Auberon Waugh to draw attention to the "crude, tasteless, and often perfunctory use of redundant passages of sexual description in contemporary novels, and to discourage it". The judges felt that far from putting Campbell off, they would risk merely encouraging him to further excesses.

The prize usually goes to painfully serious prose. Campbell is one of the few who has also written explicit erotica – which is barred from the prize – in his pre-Blairite spin doctor incarnation as a columnist for the magazine Forum.

He does have the satisfaction of beating his former boss Tony Blair to the shortlist. Many readers felt Blair should have walked it for the excruciatingly unforgettable description in his autobiography, of himself with his wife Cherie on the night of 12 May 1994: "I devoured it to give me strength. I was an animal following my instinct."

The judges did consider making The Journey the first non-fiction book ever nominated, but finally concluded that the passage was too brief to merit it. The other contenders for this year's award included the hugely-acclaimed Freedom by Jonathan Franzen, Booker prize shortlistee The Slap by Christos Tsiolkas, and poet Craig Raine's first prose novel Heartbreak.

In 2003, the columnist and former Today editor Rod Liddle was dropped from the shortlist when some of the judges felt his sex scenes were actually rather well done. He argued that they were unqualified to assess this since nobody on the Literary Review had had sex since 1936 in Abyssinia. He was duly reinstated, but in the event lost out to the Indian author Aniruddha Bahal, for his novel Bunker 13 – whose publishers, Faber and Faber, were so proud of the honour that they flew him over from Delhi to accept the award.

Previous winners include Rachel Johnson, for her novel Shire Hell. Johnson is now editor of the Lady, where the formidable owner Julia Budworth recently remarked: "You can never get her away from penises. I think it comes from growing up with all those boys" .