Fumes from a "bodged and botched" boiler at a holiday bungalow led to the deaths of two British children in Corfu, an inquest has heard.
Robert and Christianne Shepherd, aged six and seven, of Horbury, West Yorkshire, died from carbon monoxide poisoning in October 2006.
Engineer Thomas Magner who examined the boiler at the property said it had been incorrectly installed.
Coroner David Hinchliff described the deaths as "a most appalling tragedy".
Parents 'in coma'
Mr Magner told Wakefield Coroners' Court the boiler's fume protection safety device had been disabled and defects in the unit, which had been "bodged and botched", had "directly caused the deaths".
He said he had "never seen levels of carbon monoxide that high coming from a boiler".
But the engineer said a crucial problem was that a safety cut-off device had been deliberately short-circuited, meaning the boiler would not turn itself off.
The jury had been told that people staying in the adjacent bungalow had complained about having no hot water the day before the Shepherd family started feeling unwell.
As a result, hotel staff went to look at the boiler.
Asked by Leslie Thomas QC, for the family, whether this was most likely when the safety device was short-circuited, Mr Magner said: "It's the only conclusion I came to on the evidence available to me."
The children's father Neil and his partner, now wife, Ruth were found in a coma at the holiday bungalow.
Carbon monoxide levels
The jury of seven men and four women were told the children had complained of feeling unwell in the bungalow the day before they were found dead by a maid.
Post-mortem tests revealed carbon monoxide levels in Robert and Christianne's bloodstreams were 56.8 and 60.7% respectively - levels above 50% are fatal.
Mr and Mrs Shepherd and the children's mother, Sharon Wood, were in court to hear the evidence.
In May, the family were awarded legal aid for the inquest after their initial application was rejected.
Coroner David Hinchliff said the children died on "what should have been a happy half-term break".
He informed the jury the hearing is a fact-finding inquiry, adding: "The family of these children have waited a long, long time for this day to come."