A woman suffocated her two young children and placed their bodies in holdalls in the boot of her car following the breakdown of her relationship with their father, a court heard today.

Fiona Donnison, 45, had moved out of the family home in Heathfield, East Sussex, without telling Paul Donnison six months earlier, jurors at Lewes Crown Court were told.

Opening the trial, prosecutor Christine Laing QC said it was the Crown's case that Donnison killed three-year-old Harry and two-year-old Elise Donnison in order to hurt their father in the most extreme way possible.

Ms Laing said: "This is understandably a complex case particularly in relation to the motive.

"We say that responsibility lies not as a result of any mental illness but we say as a result of her personality."

She said Donnison, who was not married to the children's father but had changed her name by deed poll, was a narcissist with an overdeveloped sense of self importance and entitlement.

The couple, who had met in 1999 when both were married to other people, had an often strained relationship which was exacerbated by issues concerning her two teenage sons who lived with them and the thousands of pounds worth of debt she was prone to accumulating.

Donnison was also jealous and controlling of her partner, and often exhibited highly manipulative behaviour, the court heard.

Ms Laing told jurors she will claim she lived in fear of Mr Donnison, who she said was prone to losing his temper and often punched walls in frustration.

"Why any of this resulted in the defendant killing the children, she had not said.

"She claims to have no memory at all of the events that led to the killing of Harry and Elise."

Donnison, who had enjoyed a successful career in financial services in the City, was made redundant in July 2009 which was a blow to her self esteem, Ms Laing said, and she also became convinced her partner was seeing someone else.

On September 1, 2009 the day after returning from a family holiday to Ireland, Mr Donnison came home from work to find the defendant had moved out, taking all four children with her.

She had left a note saying "for the sake of the children I feel I have no choice but to move out... I'm sorry, Fiona".

Ms Laing said that unbeknown to him she had been looking into renting properties since she had received a large settlement when she was made redundant two months earlier.

She would not tell him where she had gone at first but he later found out she had moved into a house in Lightwater, Surrey, 100 yards from where his first wife lived with their two children, despite having no connections to the area.

Ms Laing said the couple later reconciled and made plans to move in together again but Donnison remained jealous of a woman he had struck up a platonic relationship with after she had left.

Donnison would often refuse to let him in the house and once even threw all of his possessions out on to the driveway.

She said the two children, "described by everyone who knew them as delightful, well-mannered, affectionate children", were last seen alive on the afternoon of January 26 last year and it is believed they were likely to have been killed sometime that evening.

Donnison denies two charges of murder.

Ms Laing said: "They had been suffocated, most probably by having a pillow or some other form of bedding placed over their faces."

She said both were in the pyjamas when they were found by police officers in the boot of Donnison's Nissan car which was parked in Mill Close, Heathfield, around the corner from their former family home named Meadowside.

Ms Laing said Donnison had killed the children at her rented house in Lightwater and then driven to Heathfield, parked the car, and gone into Meadowside armed with two kitchen knives.

"We suggest the purpose of her doing so was that having informed Paul Donnison of the deaths of the children the defendant also intended to kill him and no doubt blame him for killing the children."

However she added that Mr Donnison was not at the house and did not return that night or the next morning.

At around 10.45am on January 27, the defendant went to Heathfield police station and told officers she had killed her children.

She would not tell them where they were but a search of the area soon located them to the boot of the car.

Dressed all in black and with her arm in a black sling, Donnison sat in the dock with her head bowed as Ms Laing outlined the case.

The barrister said Donnison will claim she was suffering from depression and was not in her right mind at the time of the killings, and that the charges should be reduced to manslaughter.

Earlier the original jury was discharged and a new jury of five men and seven women was sworn in.

The case was adjourned until 10.30am tomorrow when Ms Laing will continue opening the case.