The Shepherd family bungalow in Corfu

Christianne Shepherd, seven, and her six-year-old brother Robert died on Thursday at their holiday bungalow.

A pathologist said carbon monoxide poisoning was the cause of death and the investigation is focusing on the room's gas boiler and air-conditioning.

Tour operator Thomas Cook said the hotel passed its latest safety check.

It said the latest audit of the Louis Corcyra Beach Hotel showed "the property met all of the Federation of Tour Operators' requirements".

Mrs Wood visited the bungalow where her children died

The children were found dead in their hotel room next to their father Neil Shepherd, 38, and his partner Ruth Beatson, 27.

They are both in a serious condition but recovering in hospital and remain unaware of the children's deaths.

Chief of police Dimitris Bineris said the hotel owner, hotel manager, maintenance manager and an engineer who installed an air conditioning unit would be charged with negligent manslaughter once toxicology tests on the children's blood have been completed.

Under Greek law the individuals could face up to five years in prison.

Condolences

A British couple who stayed in the same hotel room four days before have said they woke up one morning feeling ill and needed hospital treatment.

Kathy Pallant, who stayed in bungalow 112 with her husband Andrew, said: "My husband's a big man, and it took an awful lot of his strength for him to crawl round and get to the telephone for help."

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Helen Atkinson, from the UK's gas watchdog Corgi, told BBC News: "Many of the regulations in other countries aren't as stringent as they are in the UK.

"What CORGI would like to do is call on all UK holiday and tour operators to make sure that the work that is carried out on gas installations is carried out by qualified engineers."

The hotel extended its condolences but said it would not comment on the cause of the children's death because it could prejudice the police investigation.

The children's natural mother, Sharon Wood, 35, who lived with her children in Horbury, near Wakefield, identified their bodies.

She then visited the hotel - carrying a large bouquet of white iris mixed with pink flowers - and spent 30 minutes at the bungalow where her children died.

Thomas Cook said it had moved remaining guests from bungalows and given all guests at the hotel "the opportunity to move to alternative hotels if they wish".

But the BBC's Luisa Baldini said many guests were staying until the end of the season, in two days.

The family had been taking a week's holiday for the school half-term break.

On Thursday morning, a cleaner at the hotel in the village of Gouvia discovered the two children dead in their room, and Ms Beatson and Mr Shepherd unconscious.