A coroner’s verdict on Bristol father John Hogan, who pushed his two children off a hotel balcony, killing his son, was flawed by a “really quite serious error of law”, a High Court judge said today.

Avon coroner Paul Forrest found that former tiler Hogan, from Bradley Stoke, “unlawfully killed” six-year-old Liam as a holiday attempt to salvage his marriage went disastrously wrong.

Today Sir Anthony May, sitting with Mrs Justice Dobbs, said of Mr Forrest’s conduct of the inquest: “Really a quite serious error of law was made.

“The question of (Mr Hogan’s) mental state was simply not addressed”.

Hugh Mercer QC, appearing for the coroner, accepted there had been an error.

The court has to decide this afternoon whether or not the unlawful killing verdict is nonetheless sustainable, or whether it should be quashed and a fresh verdict substituted – or a new inquest held.

A Greek court found Hogan not guilty of murder but ordered him to be detained in a psychiatric unit. The jury decided he had been suffering from “an earthquake of insanity”.

Hogan’s older sister, Christine O’Connor, from St George, Bristol, today asked the High Court to overturn the unlawful killing verdict on the grounds that Hogan was not responsible for his actions because of his psychotic state.

Hogan, 34,pushed Liam and his two-year-old sister, Mia, before jumping himself following a row with his then wife, Natasha.

Liam died but Mia survived the 50ft plunge from the fourth-floor balcony of the Petra Mare Hotel at Ierapetra, Crete, in August 2006.

The Hogans had gone on holiday in a “make or break” attempt to patch up their failing marriage and the incident occurred shortly before they were due to return home.

Before the balcony plunge, an argument had started between the couple and Natasha said she intended to leave her husband and take the children with her.

She has since married again and is listed as an “interested party” for the hearing.