THE niece of a British government minister has told a court she drunkenly plunged a knife into the naked body of a Frenchman she picked up in a bar, but had no idea why she did it.

Jessica Davies told her murder trial she had blacked out and had no recollection of the killing except "the sensation of the knife going in".

"I am horrified by what I have done," said the 30-year-old niece of Britain's multi-millionaire junior defence minister, Quentin Davies.

"I remember telling the police that I was a monster," she said. "I don't remember killing Olivier, but I take responsibility for killing him."

She said she no longer remembered making a statement on the day of the November 2007 murder that said: "I just wanted to cut him a little, but the knife went in by itself."

The blood-stained kitchen knife with a 15cm blade that killed 24-year-old Olivier Mugnier was passed round for inspection by the three judges and the five women and four men of the jury at the court in Versailles.

Ms Davies, looking pale and drawn and dressed in jeans and a grey top, was giving evidence on the first day of her two-day trial at which the court heard of her dysfunctional family and her troubled adolescence.

Ms Davies, who was born in London to a British father and a French mother and grew up mostly in France, faces a maximum sentence of 30 years in prison if convicted of voluntary homicide without premeditation.

She said she had begun the weekend that Mugnier died by sleeping with an ex-boyfriend on the Friday night.

She spent most of the Saturday drinking in a series of bars and cafes with friends before going alone to O'Sullivan's Irish pub near her home, where she met and began flirting with Mugnier.

They had several drinks and when the bar closed decided to go to Davies's flat in Saint-Germain-en-Laye, a chic Paris suburb.

"I opened a bottle of wine. He rolled a joint... We started undressing, making love," she told the court.

"He couldn't have a proper erection, but that was understandable given how much we'd drunk. We stopped making love. He took off the condom."

She said the next thing she knew Mugnier was lying on the bed bleeding from the neck, and she was trying to stem the blood with one hand and with the other was calling emergency services.

They arrived around 2.40am on the Sunday morning but within an hour Mugnier had succumbed to the two knife wounds, one of which went right through his body and touched his spine.

Ms Davies said the knife that killed Mugnier was the one she had used to slash her own wrists and legs in her bathtub just a few months earlier when she was depressed after splitting up with her boyfriend.

Police told the court there was no sign of a struggle in the one-bedroom flat and neighbours had heard nothing.

They said Ms Davies was four times over the drink-drive alcohol limit and had traces of anti-depressants in her blood.

Earlier the court heard how Ms Davies developed behavioural problems after the acrimonious divorce of her parents when she was 14 and how she was badly affected by her maternal grandmother's repeated suicide attempts.

She started drinking heavily as a teenager and by the time she was in university, "I was drunk six days out of seven" she said, adding that she had also developed a cocaine habit.

Her mother broke down in tears at one point as she detailed her daughter's difficult adolescent and university years.

Her uncle, Quentin Davies, hit the headlines in 2007 when he defected from Britain's Conservatives to the ruling Labour party, just before Gordon Brown was named the new prime minister.

He has since been appointed Mr Brown's junior defence minister.