A factory worker who stabbed his eight-year-old daughter through the chest in an apparent act of revenge against his ex-partner has been found guilty of murder.

William Billingham, 55, used a kitchen knife to kill Mylee Billingham after dragging her into his bungalow, moments after holding the blade to the neck of her mother, Tracey Taundry.

A trial at Birmingham crown court heard that Taundry dialled 999 from outside Billingham’s house in Brownhills, near Walsall, telling operators to hurry as Mylee was screaming: “Stop it, Daddy.”

Jurors deliberated for about 80 minutes before unanimously convicting Billingham of murder and a separate charge of making a threat to kill 34-year-old Taundry. He will be sentenced on Tuesday.

Billingham, who is unemployed, opted not to give evidence, claiming he had no memory of stabbing Mylee, and offered a guilty plea to the lesser offence of manslaughter on the grounds of depression.

Prosecutors argued that Billingham turned his anger on Mylee to spite Taundry after she began a same-sex relationship.

Opening the prosecution case at the start of the trial, Karim Khalil QC said of the killing: “It was swift, deliberate, clinical, brutal. It was not some manic unfocused assault. This was no accident and it was not a slight injury, it was a deep, violent thrust of a lethal weapon into the most vulnerable part of his young daughter’s body.”

The jurors who convicted Billingham were not told he had cut his wrists in his cell partway through his trial. Billingham underwent surgery after the apparent attempt to end his own life in the early hours of 21 September – a day after his trial was shown CCTV footage of Mylee in a shop on the night he killed her.

The defendant looked at the floor and covered his eyes with his hands as the two-minute video was shown to jurors on the third day of his trial. He then failed to appear in court until the following week, when jurors were told to disregard the bandages on his lower arms and were given no details of how he had come by his injuries.

In legal argument that could not be reported until the end of the trial, the defence barrister David Mason QC said seeing CCTV of Mylee had badly affected Billingham, who spent four days in hospital.

On the morning Billingham was admitted to the Queen Elizabeth hospital from HMP Birmingham, Mason told the court his client had experienced “some sort of breakdown” overnight, during which he was injured.

Billingham was eventually brought back to court on Tuesday 25 September, arriving in a taxi with a police escort as his wounds meant he could not be handcuffed or placed in a prison van.

After Billingham was assessed by a psychiatrist as being fit to instruct his legal team, the trial continued.

• In the UK, Samaritans can be contacted on 116 123 and the domestic violence helpline is 0808 2000 247. In Australia, the crisis support service Lifeline is on 13 11 14 and the national family violence counselling service is on 1800 737 732. In the US, the suicide prevention lifeline is 1-800-273-8255 and the domestic violence hotline is 1-800-799-SAFE (7233). Other international helplines can be found at www.befrienders.org.