Gavin Hall killed Millie, 3, as her mother and younger sister slept upstairs. Two days earlier he had used chloroform to kill two of the family’s three cats and hidden their bodies in the garden shed.

After killing his daughter, Hall placed the dead pets and Millie’s teddy bear next to her body on the living-room floor before setting out to commit suicide. The next morning his wife, Joanne, found him on the sofa when she brought their younger daughter, Lucy, downstairs. He was unconscious and bleeding heavily from cuts to his neck, legs and arms.

Hall, who sent text messages to his wife and her lover to explain his actions, was treated in hospital. He later told the police: “I killed my angel and I have not joined her.”

The harrowing case was outlined at Northampton Crown Court, where Hall, 33, from Irchester, Northamptonshire, denies murder but admits manslaughter on the ground of diminished responsibility.

As Hall broke down in the dock, William Coker, QC, for the prosecution, told the jury that he had discovered that his wife was having an affair with a man she had met through a sex contact website. “His anger and bitterness, his desire to punish Joanne, are obvious.”

Mr Coker went on: “The defendant admitted that he meant to kill Millie. It is no defence to murder that he intended to kill himself.”

The couple met as students in Northampton in the late 1990s. Hall was a radiographer at Northampton General Hospital and his wife a coronary care nurse in Bedford.

Miss Rainsley, 31 — she has reverted to her maiden name — said that the marriage had been in trouble for some time. “There were frequent problems,” she said. “We knew it was a marriage that was not going to last.”

Describing the relationship between father and daughter, she said: “They were very close, right from the day she was born. He adored her and she thought the world of him.”

In the weeks before Millie’s death last November, Mr Coker said, Hall had been off work sick and had told friends he had been contemplating suicide. He had learnt about his wife’s affair when a letter arrived from a hotel into which she had booked to have sex with her lover, identified in court only as James.

Hall had smashed her mobile phone and demanded to read her e-mails to find out about her contacts with the man, a solicitor and part-time judge.

On the night of Millie’s death, Hall changed her from pyjamas into her favourite nightdress and gave her some of his antidepressant pills to make her drowsy. After cuddling her for more than an hour, he held a chloroform-soaked rag over her mouth for more than 20 seconds until she was dead.

In her evidence, Miss Rainsley said that she felt that she had been trapped in a loveless and sexless marriage. Her lover, who was married with children and whom she had met twice for sex in hotels in Northampton, had posted a picture of himself naked on the internet and they had corresponded by e-mail.

The case continues.

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