A mother killed her two young children in an act of revenge against their father, a court was told.

Fiona Donnison, 45, was described as a “narcissist” who could not accept that her relationship had broken down and that her estranged partner had started seeing another woman.

She is alleged to have killed her children, Harry and Elise Donnison, at her rented home before putting their bodies into sports holdalls, placing them in the boot of her car and driving to the house in Heathfield she had shared with Paul Donnison, a jury at Lewes Crown Court was told yesterday.

She then waited inside the property in Sheepsetting Lane, Cross In Hand, Heathfield, with two kitchen knives she had brought with her, intending to kill Mr Donnison and blame the children’s deaths on him, it is alleged.

Donnison denies murdering Harry, three, and Elise, two, between January 25 and January 28, 2010.

Prosecutor Christine Laing said Harry and Elise were described as delightful, well-mannered and affectionate children.

But during the morning of January 27, 2010, they were found in the boot of the defendant’s car having been suffocated with a pillow or something similar, dressed in pyjamas and their bodies placed in two sports bags.

The last sighting of the children alive had been the previous afternoon, the court was told.

At 11.45am the following day, Donnison is alleged to have walked into Heathfield police station and told staff she had killed her children. She did not say where the bodies were but following a police search, they were found in the boot of the car parked in Sheepsetting Lane.

Miss Laing said it had not been possible to establish the exact time of death but it is likely Harry and Elise were killed at Donnison’s rented home in Lightwater, Surrey, the night before.

She said: “The defendant then drove to Sussex with the bodies in holdalls in the boot, hid her vehicle near the old home in Sheepsetting Lane and entered the property with two large kitchen knives.

“Having informed Paul Donnison of the deaths of the children, she was going to kill him and blame him for killing the children.”

But Mr Donnison did not return home that night and the next morning went straight to work in Berkshire. The court was told that on September 1, 2009, the defendant left her husband without warning and moved herself, Harry and Elise to the rented property in Lightwater.

Soon after Mr Donnison attempted a reconciliation but by January 2010, the family home was on the market and Mr Donnison was in a relationship with someone else, the court was told.

Miss Laing said Donnison had long lived beyond her means, racking up £50,000 to £60,000 of debt before she began a relationship with Mr Donnison in 2002.

They moved in together and Mr Donnison agreed they would downsize to pay off her debts.

Miss Laing said: “The defendant was unhappy with that because material things, having the best and keeping up with friends’ standards concerned her a lot.

“Paul Donnison described her as someone who always wanted bigger and better things than they could afford.

“She worked in the City and would often remind Mr Donnison she earned more than him.”

The court was told that the defendant and Mr Donnison had never married but after Harry was born Donnison had changed her name by deed poll.

On January 14, 2010, the defendant told Mr Donnison she could see no future in their relationship and he agreed with her, the court was told.

Miss Laing said: “We say it was that conversation that was the catalyst to the terrible tragedy that was to follow.”

Earlier, as Miss Laing opened the case, she told the jury: “The Crown says she simply could not accept this rejection of her and that the killing of the children was an act of retaliation or revenge against the father intending to cause him the maximum hurt possible.”

Donnison denies two counts of murder. The trial continues.