Grenadier Guardsman Daniel Crook jailed and dismissed from the army after bayoneting boy, 10, in kidneys for no reason

A British soldier has been dismissed from the army after stabbing a 10-year-old Afghan boy in his kidneys with a bayonet for no reason.

Grenadier Guardsman Daniel Crook was suffering from a hangover after a heavy vodka drinking session when he bayoneted the boy, who was running an errand. He could not explain why he carried out the attack.

After being traced by the Guardian, the boy's father said the attack had left his impoverished family bitter and financially burdened. More than 18 months after the attack, his son is still unable to go to school. He said British forces were "in Afghanistan to build the country and remove insurgents, not to stab a child".

The unpublicised conviction of Crook is the latest in a series of prosecutions mounted against British military personnel accused of causing civilian casualties in Afghanistan.

The Guardian has learned that in a separate case, another soldier is being investigated on suspicion of murder after allegedly shooting dead an Afghan civilian who was digging near a military base.

Prosecutors are considering whether to charge the private in the Royal Welsh Regiment over the incident in Helmand province in June last year.

The Guardian has pieced together the the court martial of Crook from an interview with the boy's father and an account from the Service Prosecuting Authority (SPA), the agency established in 2009 to conduct independent prosecutions of military personnel.

One evening in March last year, Crook "drank a considerable quantity of vodka which was sent to him in a mineral bottle contained in a welfare parcel", according to prosecutors. He was so drunk that medics had to treat him overnight.

At 9am the next day, his unit left a checkpoint in the Nad e Ali district of Helmand to go on patrol. Crook followed his colleagues after arming himself with a pair of grenades and a bayonet. His rifle had been confiscated as a safety measure, the court martial heard.

He soon came across two Afghans riding bikes. One of them was the 10-year-old boy, Ghulam Nabi. His 72-year-old father, Haji Shah Zada, said his son had been sent to collect a bottle of yoghurt.

The soldier was "alone and not patrolling and he ordered Ghulam to stop. But he was just a little child and he didn't understand. The soldier had just come out of his base and he didn't have a gun, only a bayonet. He grabbed Ghulam and stabbed him in the back, near his waist."

Prosecutors said the boy had pesteredCrook for chocolate. "In response Crook took hold of the boy's shoulder and stabbed him in the region of his kidneys with his bayonet. Crook felt the bayonet pierce the boy's skin but did not see if he was bleeding." His father said: "His clothes were covered with blood. We put him in a car and raced to Lashkar Gah but the doctors couldn't treat him there and we took him to Kandahar."

Meanwhile Crook had caught up with his patrol and admitted he had stabbed a child. The bayonet and grenades were taken from him and he was handcuffed. He later told military police he could not explain why he stabbed the boy.

At the court martial in June, the judge described the trigger for the crime as "the considerable amount of alcohol consumed the night before" and ruled that he had put the lives of his comrades in danger. Crook was jailed for 18 months and dismissed from the army.

This week, the boy's father said he had a big scar on this back from the wound. He boy is still not going to school as he struggles with the 1km walk and riding his bike is impossible.

The attack has imposed an extra financial burden on the poor family as Ghulam cannot run errands or help his father collect grass for their animals.

Shah Zada, a shopkeeper and farmworker, says he cannot understand why his son was attacked and has received no apology from the British forces. Although he credits Nato troops for expelling the Taliban from his village, the attack has soured his opinion of western forces. "Of course foreigners are the enemies of Afghans – otherwise he wouldn't do that to innocent child who was just going by on his bike." On the question of compensation, he said: "We asked for $40,000 but they only gave us $800."

Six members of the British armed forces have been, or are being, prosecuted over allegations of abusing or wounding Afghan civilians since March last year.

The prosecutions, which have led to four convictions, were brought by the SPA , headed by Bruce Houlder QC, a former chairman of the Criminal Bar Association. Ministers were forced to set up the civilian-run agency following severe criticism that military prosecutors had failed to properly prosecute British troops accused of killing or abusing Iraqi civilians.

Details of the prosecutions are deeply embarrassing given that foreign troops in Afghanistan say their mission is to build up a stable and lawful country. Civilian casualties play an important role in the propaganda battle to win the "hearts and minds" of Afghan civilians – waged by both Nato and the Taliban.

In a separate prosecution which was reported at the time, two Royal Marines were dismissed from the navy for assaulting an Afghan civilian. Sergeant Mark Leader and Captain Jody Wheelhouse assaulted Mohammad Ekhlas, 48, with a boot. He needed four stitches to his lip.

Michael Hunter, the judge at the court martial, ruled that "this was a sustained assault on an injured and unarmed prisoner". He added that it was understandable that soldiers who have seen the effects of roadside bombs should "feel a degree of hatred towards those who plant them", but assaulting prisoners undermined the standards of the armed forces and everything they were trying to achieve in Afghanistan.

In a further prosecution, Dane Keir Leyland-Jones, of Ist Battalion, the Duke of Lancaster's Regiment, was fined £1,000 in May for negligence after an Afghan policeman was shot. According to the SPA, he was manning a checkpoint in Nad e ali in May last year when a bullet from a gun he was handling struck the left leg of the policeman. Medics decided that they needed to amputate his lower leg, but he vanished and has not been found since.

The SPA is also prosecuting a corporal, understood to be in the special forces, for allegedly assaulting an Afghan in March last year.

Many Afghan civilians have been killed or injured by British forces during the decade-long conflict, although United Nations figures show that the Taliban kill far more civilians than Nato troops.

Britain's military police investigated 99 incidents in which UK forces had been accused of killing or wounding Afghan civilians between January 2005 and March this year.