A mother accused of murdering her three-year-old daughter said she drowned her in the bath to "save her" after separating from her executive husband who she feared was having an affair, a court has heard.
Claire Colebourn is accused of drowning Bethan at their home in Fordingbridge, Hampshire, after the 36-year-old split with husband, Michael, the CEO of luxury marine interior company Trimline.
Kerry Maylin, prosecuting, told Winchester Crown Court Bethan was found by her grandmother, Janet Fildew, when she arrived for a visit to the family home in Whitsbury Road on October 19 2017.
The lawyer said: "Bethan was three-years-old, she was a much loved daughter of Claire and Michael Colebourn, but she was found dead at her home.
"Claire Colebourn, when Bethan was found, was also in the house and she was at the time suffering a diabetic episode.
"Bethan died because she had been put in the bath at home and held under the water, the act was completed by her mother."
Miss Maylin said that in a nine-and-a-half-hour period that day, Colebourn had administered a dosage of insulin 10 times that she would normally use in a whole day.
The prosecutor said that just over a month before Bethan's death, Colebourn and her husband, who had been together for 16 years and married for six years, separated and Mr Colebourn had moved out.
Miss Maylin said their relationship had been difficult and the defendant went on to make "unfounded accusations" on Facebook and an email sent to his work, that her husband was having an affair with the company's finance director Kelly Futcher.
In the email to Trimline, she wrote: "He has been having an affair with his finance director at work, everything has been pre-planned, of course the finances.
"They are aiming to take over the business and set up a new life together."
Miss Maylin said Colebourn also told friends she had visited Ms Futcher's home looking for her husband's car.
The lawyer said Colebourn also became worried, without foundation, that Mr Colebourn was monitoring her computer and arranged for an IT firm to supply her with a new laptop and wifi at her home.
She said that within minutes of receiving the new computer in October, she began searches for suicide-related websites and another website called How To Kill By Drowning, as well as watching a YouTube video titled Someone Drowns In A Tub Nearly Every Day.
On the day before Bethan's death, Colebourn also wrote a letter to her mother saying: "I love you, Bethan loves you, thanks for everything. Please make sure Walter (horse) and Charlie (dog) are looked after. Love always Claire."
She added: "Please make sure I am buried with Bethan in Fordingbridge."
Miss Maylin said that following her arrest, she told medical staff she had tried to kill herself the previous day.
The prosecutor said a doctor described how she was "only emotional when discussing the fact Michael Colebourn had left her and her daughter and her father had done the same to her mother".
She later wrote in a letter intercepted at a hospital she was staying in: "In my eyes I saved her, everything over those days is a blur."
She later told police: "I am responsible for Bethan's death because she drowned and I am responsible for it. Bethan drowned because I was there, I held her under the water."
She also made accusations that her husband had been "controlling and a bully".
Miss Maylin said the last time Mr Colebourn saw his daughter alive was when he took her to a soft play centre on October 15 and said: "He and Bethan had a nice afternoon and when he left she hugged and kissed her father goodbye."
Colebourn denies murder and the trial continues.