A woman who doused a bed in petrol and set it alight while her ex-lover lay naked with her new girlfriend today said her heart ''dropped to her socks'' when she saw the pair caressing.

Sarah Metcalfe, 46, of no fixed abode, discovered Katie Wrigglesworth in bed with Emma Shakesheff before the incident on July 20 last year, Leeds Crown Court heard.

She denies causing grievous bodily harm with intent during the attack in a caravan which she owned at Goldsbrough Mill Farm, near Knaresborough, North Yorkshire.

She pleaded guilty today to an alternative charge of causing grievous bodily harm, saying it was not intentional.

Breaking down in tears, Metcalfe said: ''I saw Katie and Emma sat at the end of the dining table caressing. Katie was naked and Emma was wearing her pants.

''Seeing them in a state of undress and in an embrace, it's almost like your heart drops to your socks.

''I wish I had turned round and walked out.''

Instead, she said, she stayed and started bickering with Miss Wrigglesworth while Miss Shakesheff went to the bedroom.

''She was being awkward so I was being awkward,'' Metcalfe said.

After bickering for around 10 minutes over their cat, Blue, Metcalfe said Miss Wrigglesworth told her: ''I've had enough of this, I'm off to bed to make love to my bird.''

Metcalfe told the jury of seven women and five men that she sat alone for a few moments and felt lost and as though she had nothing left.

As she got up to leave she glanced through the open bedroom door and saw the pair together.

Sobbing, she said: ''I glanced and saw Katie on top of Emma appearing to be in the process of love-making.

''I can't say what I felt when I saw that. I just wanted them to come out the bedroom and stop. I don't know what possessed me.''

She said she turned round and picked up a can of petrol that was stored in the bunk bedroom for use in the caravan's generator.

She said: ''I just wanted them to get off the bed so I poured the petrol.

''There didn't seem to be any reaction from them. I flicked the lighter and that's when I got a reaction.

''Katie said I hadn't got the balls to do it.

''I went to flick the lighter again expecting the same as it had done the first time. But there was this wall of flame. I remember jumping back. ''Then I realised what had happened and what I had caused.''

Earlier, the court heard that Metcalfe told police she ''snapped'' when she saw the pair making love and flicked the lighter.

She said she didn't want to kill or hurt either of them, but had just wanted them to stop making love. The jury of seven women and five men heard the pair were initially trapped and suffered serious injuries in the blaze, but escaped through a window.

Ms Wrigglesworth, 26, suffered 40% partial thickness burns to her arms, legs and body and had to spend 10 weeks in hospital.

Ms Shakesheff, 32, suffered extensive second degree burns to 44% of her body and spent six weeks at Pinderfields Hospital in Wakefield, the court heard.

Metcalfe wrote to Miss Wrigglesworth, who was still recovering in hospital, while she was on remand in prison.

In the letter, she said: ''I never meant to hurt you. I just wanted you to stop making love to her in front of me. It felt like you were ripping my heart out with your bare hands.''

Metcalfe and Miss Wrigglesworth had previously had a relationship lasting around four years while they were living at Cold Bath Road, Harrogate, but the pair broke up in June last year and used the caravan after the house had been sold.

Miss Wrigglesworth had subsequently started seeing Miss Shakesheff.