The judge said it was not a case where they were “inadequate people incapable of caring properly”, but had instead chosen to be “spiteful and at times sadistic” to the defenceless little boy.

He said he had not seen a shred of remorse from either of the killers. However, under the current legal framework, he said the starting point for Tustin’s appalling crime was 30 years, rather than the whole life tariff which can be handed out to child sex killers.

The lack of obvious premeditation in the murder means that such an offence would not qualify for a whole life term when changes are introduced as part of the new Police, Crime, Sentencing and Courts Bill.

On Friday, there were calls for Dominic Raab, the Justice Secretary, to address this loophole to ensure people who torture and kill children are sent to prison for rest of their lives, regardless of whether the murder was premeditated.

Tim Loughton, former Tory children’s minister, said: “Undoubtedly the law should be changed. When do you judge if serious abuse has become premeditated?”

Tustin was convicted of a count of child cruelty by poisoning the six-year-old boy with salt. Mr Loughton warned that could have killed Arthur, rather than physical blows that led to his fatal brain injury.

“The premeditated condition could get in the way of [punishing] the systematic onslaught of abuse and torture, in this case against Arthur,” said Mr Loughton

“This lot deserve to be thrown in jail and the key thrown away. It shouldn’t ride on a legal technicality.”

Sir Mike Penning, former justice, policing and victims minister, added: “Anybody with an ounce of common sense would think that’s what the legislation should mean. If it is not, then Parliament should act immediately to close this loophole.

“This type of abhorrent crime doesn’t happen often but when it does, I cannot see why anyone should be let off with anything less than a whole life sentence.”

‘Manipulative woman who would tell any lie’

Tustin, a mother of four, who was pregnant with Hughes’s baby when she killed Arthur, refused to attend court for the sentencing hearing.

Addressing her in her absence, Mr Justice Wall said: “You are a manipulative woman who would tell any lie and shift the blame on to anyone to save her own skin.”

He said he was sure she had intended to kill Arthur when she launched her final attack on him last June, because she blamed him for any problems in her life.

Addressing Hughes, 29, he said: “You were Arthur’s father, in a position of trust and bore primary responsibility for protecting him.”

But he said instead he had “relished to make him suffer” and described his “encouragement” of his girlfriend’s actions as “chilling”.

Meanwhile, the father of one of Tustin’s other children said she had no maternal instincts whatsoever and instead preferred to spend her benefits money on tattoos, clothing, jewellery and getting her hair done.

After the birth of her first child, she threw herself out of a bedroom window following a row with her boyfriend. Eight years ago, she jumped off a multi-storey car-park in the West Midlands, spending five months in hospital.

Her two eldest children later went to live with their fathers, but her two youngest were allowed to stay with her at her home in Shirley, Solihull.

The judge said Tustin had shown copious amounts of love to her own two children, adding: “It is a shocking feature of this that your children lived a perfectly happy, normal life in that household while this appalling cruelty to Arthur was taking place”.