Gavin Hall, who killed Millie, 3, at the family home before attempting suicide as his wife and younger daughter slept upstairs, will serve a minimum of 15 years in jail, Judge Charles Wide, QC, said.

Hall, 33, told police: “I killed my angel and I have not joined her.” He was convicted at Northampton Crown Court after a six-day trial.

Outside the court Joanne Rainsley, 31 — she has reverted to her maiden name — paid tribute to her daughter.

She said: “My children, Millie and Lucy, were our pride and the love and joy of our lives. It is incomprehensible to us as to how anyone under any such circumstances could deliberately take away such a beautiful little girl as Millie.

“Millie will always be remembered as a happy, lively and beautiful little girl and I am proud to have been her mum.”

During the trial, the jury was told how Hall had initially painted a picture of a double suicide attempt, “like a scene from Romeo and Juliet”, in which Millie agreed to “come with Daddy”.

In rambling letters, littered with quotes from Shakespeare, he told his wife: “Do not lament Millie, she needed me more. What little love you do show can now be for Lucy [the couple's younger daughter]”.

He claimed that Millie had willingly taken one of his antidepressants before falling unconscious from the chloroform. But the jury was told that Hall had admitted that scratches on Millie’s face were caused by his fingernails as he forced the rag over her mouth. He strangled her with his left hand as she lay dying in his arms.

As Hall sobbed, Judge Wide told him that Millie’s murder was a planned and premeditated attack carried out after he discovered “extraordinarily sexually explicit” e-mails be-tween his wife and her lover, who was named last night as James Muir-Little, a divorce lawyer who recently became a deputy district court judge.

Judge Wide told Hall: “It has been said, and it is true, that you will have to live for the rest of your life with what you did. Joanne will have to live for the rest of her life with what you did.”

Two days before he murdered Millie in November last year, Hall, from Irchester, Northamptonshire, used chloroform to kill two of the family’s three cats and hid their bodies in a shed. He placed the dead pets and Millie’s teddy bear next to her lifeless body on the living-room floor before attempting suicide.

The next morning his wife, a nurse, came downstairs with Lucy to find him unconscious and bleeding heavily from cuts. Millie was on the floor.

The court was told that Hall suffered an “emotional disturbance” after reading the e-mails between his wife and Mr Muir-Little.

Ms Rainsley met the 45-year-old judge in September last year through a sex contact website for married people.

Nicholas Atkinson, QC, for the defence, said that Hall discovered the relationship a month later.

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