A Polish mother and her partner who systematically starved her four-year-old son before savagely beating him to death were jailed for a minimum of 30 years by a British judge on Friday.

Magdelena Luczak and Mariusz Krezolek subjected Daniel Pelka to a six-month ordeal in which he was held underwater, locked in a room and force-fed salt before suffering fatal head injuries in a vicious assault.

The case has raised questions about why Daniel's stepfather Krezolek was not deported to Poland despite having been jailed three times in Britain, and about the failure of Daniel's school and social services to save him.

The case was "deeply shocking and disturbing" and involved "unimaginable acts of cruelty and brutality," said Judge Laura Cox as she sentenced the pair at Birmingham Crown Court.

Cox gave Luczak and Krezolek each life sentences and said the couple, who were found guilty by a jury on Wednesday, must serve a minimum of 30 years in prison before they can be considered for release.

The pair both arrived in Britain from Poland in 2006.

A juror wiped away tears as the sentences were read out. Luczak and Krezolek, who are both originally from Poland stared straight ahead and listened to their interpreters.

Daniel was described as a healthy little boy when he started school in the industrial city of Coventry in September 2011. Pictures showed a smiling blond-haired boy in a red uniform.

But the time he died in March 2012 experts "likened his appearance to those who failed to survive concentration camps, and the comparison was not made lightly," the judge said.

His mother, 27, and her partner, 34, who were both drug and alcohol abusers, subjected him to multiple beatings and repeated punishments.

They repeatedly poured neat salt into his mouth, causing him to vomit.

Daniel was also subjected to a "cold water punishment" in which he was held under water to the point of unconsciousness, the judge said, adding that the boy's six-year-old sister had witnessed it.

The couple regularly locked the little boy inside a bare box room with no toys, removing the inner door handle so he could not escape and making him to go the toilet on his bed.

But the judge said the "particularly grave aggravating feature" of the case was the "chronic and systematic starvation" that they subjected him to.

Daniel's sister smuggled him food because he was so hungry, while the little boy was forced to scavenge for food from other children's lunch boxes or from rubbish bins.

The judge said Daniel died after a "brutal assault" by shaven-headed Krezolek, just after the child was again given salt and held beneath the water.

The pair failed to call for an ambulance for 33 hours, during which time they looked on the Internet for medical advice as he lay unconscious in his box room.

An autopsy found 23 injuries on his emaciated body.