A man who murdered his partner’s baby son by striking his head against a hard surface in “sudden temper and frustration” has been sentenced to life in prison with a minimum term of 18 years.

Kane Mitchell, 31, fatally assaulted Teddie Mitchell in November 2019 while they were alone together, when the infant’s mother Lucci Smith, 29, was on the morning school run in St Neots, Cambridgeshire.

Teddie died in hospital 10 days later on November 11 2019, aged 12 weeks.

Judge Mr Justice Knowles said at Cambridge Crown Court on Friday that Mitchell, who is not the infant’s father, squeezed Teddie with so much force that he fractured the boy’s ribs and his collar bone.

He said that the assault on November 1 also involved the “violent striking by Mr Mitchell of Teddie’s head against a hard or unyielding surface”.

This fractured the boy’s skull and caused brain, spinal and eye injuries, the judge said.

He said that on a separate, earlier, occasion Mitchell had fractured two of Teddie’s ribs and his collar bone.

He found that Mitchell did not intend to kill the child but that “in sudden temper and frustration with Teddie” he intended to “cause really serious harm” to him.

Matthew Jewell QC, mitigating for Mitchell, said the “spontaneous loss of temper” may have been in response to the child crying, adding that it was not a case where there was an “ongoing catalogue of neglect”.

Mitchell, of no fixed address, was found guilty at trial of murder and of causing or allowing serious harm to a child.

He was sentenced on Friday to life in prison and must serve at least 18 years before he can be considered for parole.

The judge said that Smith, the child’s mother, did not call an ambulance until 3pm on November 1, after the afternoon school run.

He said: “Her failure to obtain earlier medical help couldn’t have saved Teddie – it could have spared him some pain and suffering.”

He said that Teddie’s injuries were as a “direct result” of Mitchell’s actions and that Smith “was unaware” of what happened, but that “it was obvious to her something was seriously wrong”.

There is evidence that Smith “cared about… and loved” Teddie, and that Mitchell was an “overbearing partner”, the judge said.

Smith, previously of Pattison Court, St Neots, was found guilty of cruelty to a child, for not securing earlier medical help.

The judge sentenced her to a two-year community order, noting that she had already spent several months in prison while remanded in custody ahead of her trial.