A callous mother and stepfather who battered to death a four-year-old boy after subjecting him to six months of systematic starvation and "incomprehensible" cruelty have been found guilty of murder.

A jury at Birmingham crown court convicted Magdelena Luczak and Mariusz Krezolek of killing Daniel Pelka after hearing how he was denied food, regularly imprisoned in a locked room, force-fed salt and made to perform arduous punishment exercises.

The boy's murder, which occurred weeks after teaching staff saw him with bruising to his neck and black eyes, is the subject of a serious case review by Coventry's safeguarding children board.

When his body was examined, experts were shocked at what they saw. Skin was hanging in folds off his thighs and arms. His tummy was tiny, his rib cage protruding and his spine was clearly visible all the way down his back. His hair was falling out and patchy.

One child protection expert who examined the boy said she had seen such emaciation only in pictures of concentration camp victims. A radiologist compared Daniel's frame to that of a seriously ill cancer patient, and a police detective said he looked like child from a famine-ridden part of Africa.

The final attack came in March last year when Daniel was brutally beaten. A postmortem revealed he died of bleeding and swelling in the brain. He had 10 bruises on his head, neck, shoulders and a cluster at the base of his spine.

The jury heard that staff at Daniel's school had noticed that something was wrong. He began to scavenge for food from bins and other children. He would take large amounts of fruit provided by the school, hide it and eat it in the toilets.

Teachers began to lock other children's lunchboxes away because he stole food from them. He would pick pear or apple cores out of bins and eat them. One teaching assistant told the court that by the start of 2012 he was "wasting away", his hair untidy and his uniform too big for him.

His mother and stepfather face lengthy jail terms. An inquiry will be launched to find out why the many professionals, including his schoolteachers, who had contact with Daniel and his family did not realise what was going on behind the closed doors of the family's flat in Coventry.

In a brief statement released by police, Eryk Pelka, Daniel's biological father, said: "It's a great tragedy that such a little angel had to leave this world. I hope that those responsible will be punished severely."

Neither Krezolek nor Luczak, who will be sentenced on Friday, showed any obvious emotion as the guilty verdicts were returned after around four hours of deliberation.

The nine-week trial heard the couple, both originally from Poland, hid the horrifying abuse by claiming Daniel had an eating disorder.

Jurors were also told how Daniel was left to die in his unheated "cell" for around 33 hours after suffering a fatal head injury at his Coventry home.

Former soldier Krezolek — described in court as a heartless "monster" — and Luczak blamed each other for Daniel's death during the trial.

When he died Daniel weighed only 1 stone 9 lbs, about the weight expected for an 18-month old child.

Outside court a spokeswoman for the crown prosecution service said Daniel had been condemned to live in a tiny box room with no bed and no toys. He also had to use the room as a toilet.

Police said they did not imagine what had led his mother and step-father to treat him as they had.

Amy Weir, chair of Coventry safeguarding children board, said: "The trial has shown how Daniel died at the hands of two cold-hearted and violent people — the two people whom Daniel should have expected to protect and look after him. Instead they subjected him to a cycle of cruelty and violence while aiming to deceive everyone else.

"Alongside the police investigation the Coventry safeguarding children board has been carrying out a serious case review into Daniel's death."

Gill Mulhall, head teacher at Little Heath primary school said: "Although he only attended our school for six months, Daniel will be remembered by everyone who knew him during his all too brief time here. A quiet, unassuming boy, Daniel was accepted and liked by his classmates and has been greatly missed."

Background to the case

The investigation into Daniel's death revealed he had suffered a truly awful campaign of deprivation and violence at the hands of his mother, Magdelena Luczak, and stepfather, Mariusz Krezolek.

In the months leading up to his murder Daniel was systematically deprived of food, apparently subjected to a form of water torture, forced to carry out a demanding exercise regime and beaten. When he was suspected of sneaking extra food he was made to eat salt so he would vomit it up. He was not allowed to go to the toilet at home and had to sleep on a soiled mattress in a box room.

Polish nationals Krezolek, 34, and Luczak, 27, arrived in the UK from eastern Europe independently in 2006. Luczak had Daniel by the Polish lorry driver Eryk Pelka but he left the UK in 2008, apparently after tiring of their tempestuous relationship.

He claimed she would often hit him and on one occasion he called the police after she threatened him with a knife when they were drunk at a party. He said the relationship finally broke up when she cheated on him with one of his friends and he left the UK.

At that time, Pelka said, his son was normal and healthy. Even the boy's Polish granny, who was sceptical about the nutritional value of British milk, had to concede he was chubby and well.

Towards the end of 2009 Luczak met Krezolek, a former soldier who was working on a car factory production line in the Midlands, and the pair began a relationship early in 2010. It too was a stormy, sometimes violent partnership.

Krezolek told the jury at Birmingham crown court that Luczak would "explode" when she had been drinking and smoking marijuana and was jealous of any friendships he had with other women.

Luczak claimed that Krezolek, who spent three periods in jail for repeatedly ignoring a driving ban, was violent to her, once strangling her and pulling her hair. She also claimed he raped her on the day Daniel died.

Krezolek never got on with Daniel, another man's son. In January 2011 he broke the boy's arm and left him in agony for 12 hours before seeking medical help. He explained away the injury by saying it had happened during a game of hide-and-seek.

Still, by the time Daniel started primary school in Coventry in September 2011 there was little outward sign anything was wrong. He was quiet and small but had a cheeky side. His hair was smart, and his uniform fitted him well.

Within months staff noticed that something was wrong. One teaching assistant told the court that by the start of 2012 he was "wasting away", his hair untidy and his uniform too big for him. He felt like a "bag of bones" and his skin was "ashen-coloured". He had dark circles under his eyes.

Another teacher said he looked like "an old man" and was so pale "you could almost see through him". She added that he "looked so sad, lonely, desperate".

In the early hours of Saturday 3 March last year Luczak called an ambulance to the family home in Coventry. Daniel was unconscious and could not be revived.

The couple claimed he had complained of chest pains the previous day but had gone to bed as normal. But the state of Daniel's body prompted police to launch a murder inquiry.

The pair's story quickly fell to pieces. Officers examined their computer and found that in the hours before they called an ambulance they had searched for terms including: "when a child swallows too much table salt" and "care patient in a coma".

Detectives discovered texts exchanged between the defendants that hinted at the dreadful regime Daniel had been subjected to.

One sent by Krezolek said: "Take him to the room and lock him in there. We will have some peace." Another sent by Luczak said: "We will deal with Rudey [Daniel] after school. He won't see grub at all."

Yet another from his mother said: "Well now he is temporarily unconscious because I nearly drowned him." His mother also texted: "If I hear when he wakes up he will be going back to the bathtub. I didn't let the water out."

Perhaps the most startling testimony came from Daniel's sibling, whose name, age and gender cannot be revealed for legal reasons. The child told police that Krezolek would prevent Daniel from getting food and would hit him when he was drunk and put him into cold baths. The sibling would step in to try to stop the attacks and find food for Daniel.

"I thought I needed to be brave to help my brother," the sibling said. The youngster would clean up after Daniel when the little boy soiled himself or his bed. The child said Daniel was not even allowed to use the toilet.

Describing the night Daniel died the sibling said: "I tried to wake him up but I couldn't. Then I tried to listen for his heartbeat but I could not hear anything."

Though at one court hearing the couple held each other's hands in the dock across a police officer, in the end they sought to blame each other for what happened to Daniel.

Krezolek admitted he made the youngster run around the living room, kneel for long periods and do squats when he disobeyed the couple, but said he did this at the behest of the boy's mother. Shockingly, he also admitted in court that he lay Daniel's dead body next to the boy's sibling.

Asked who had dealt the fatal blow, he replied: "Probably Magda."

Luczak claimed her partner twice hit Daniel around the head hours before he died as he was angry because the youngster had eaten extra food at school.

She claimed her partner had shouted: "This is your fucking son not mine. Send him back to Poland."

In the witness box she admitted telling Daniel's teachers not to give him extra food and bribed her son not to inform staff of his ordeal by giving him a chocolate bar. She said she had neglected him but not starved him and had not beaten him.

The couple's lies mean nobody can know for sure who struck the final blow, and nobody can be certain why they abused Daniel. Police and prosecutors believe his death was caused by a "team effort" to starve, beat and torture him.