Neil Davies, 53, stabbed his son Joshua, 24, in the neck with a kitchen knife when the two argued at their Cardiff home a few hours after celebrating a Wales' rugby victory over England in March.
Cardiff Crown Court heard the force of the blow slashed Joshua's carotid artery and almost severed his jugular vein - leaving him sprawled on a sofa soaked in blood. Despite frantic efforts by police and paramedics they could not save six-footer Joshua.
Davies was jailed for five years after he pleaded guilty to manslaughter. A neighbour in a nearby flat overheard Davies crying, "He's dying, what have I done, what have you done?" and when he rang emergency services a distraught Davies said, "I just stabbed my son, please help".
The court heard the pair had been drinking for almost 12 hours at the Cross Inn pub and the British Legion club in Rumney, Cardiff. CCTV footage at the club showed the couple pushing each other and arguing shortly before they left just after midnight to return to their flat in nearby Llanrumney.
Defending lawyer David Aubrey QC told Cardiff Crown Court that Joshua had continued with his confrontational behaviour at the flat. Mr Aubrey said, "He struck the defendant on the left cheek and his father was trying to calm him down.
"He was making sandwiches and Joshua followed him into the kitchen, The defendant had the knife in his hand preparing a meal. He is adamant he did not deliberately arm himself with the knife but accepts he must have waved it around to ward off Joshua."
The court heard that Davies had string of 41 convictions including some for violence and had served previous prison sentences.
Joshua's mother Karen Brady, 47, broke down in tears the case and had to be comforted by friends afterwards.
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The former Wigan and Great Britain rugby league star previously enjoyed a successful club coaching career with Wasps.