Seven-year-old Zane Gbangbola, was poisoned to death after floodwaters engulfed the family’s home in Chertsey, Surrey, during the worst storms to hit Britain in 248 years in February 2014.
For months after the tragedy, police and other agencies ruled out his parents’ fears that the lethal gas had come from a nearby landfill site, and insisted carbon monoxide from a faulty pump hired by the family was to blame.
He had also represented the West Yorkshire family of Bobbi and Christi Shepherd who died from toxic gas inhalation from a faulty boiler at the Louis Corcyra Beach Hotel in Corfu in 2006 following negligence by Thomas Cook.
Mr Thomas read from the Environment Agency’s National Incident Recording System report that some information in the document was to be "vetted" not to be "in the public domain".
Mr Thomas said he suspected the report "had been redacted".
Mr Harris, representing the Environment Agency, said that this had been a "formatting error" and that they "had not sought to withhold any information" from the court.
Mr Thomas read on that when the fire service entered the house in February last year after the family was taken to hospital, a gas reading of 25,000 parts per million was taken, showing "high levels of hydrogen cyanide in the upstairs bedroom".
Another recording taken downstairs had the reading 10,000 parts per million, the inquest heard.
At a previous pre-inquest review into Zane’s death, it was said that only one reading of 10 parts per million had been recorded in the house following the evacuation.
Mr Thomas said: "This report says there was more than one reading.
"It’s a very high reading. I don’t think we’ve had an indication that the hydrogen cyanide was ever so high.
"This is the first time that this has come out."
Mr Thomas added “there may be more evidence than we have seen” after the report showed that the matter was taken “extremely seriously” when Government’s Cobra emergency committee meetings were held in relation to Zane’s death.
He told coroner Richard Travers: "This is an indication that there well may be evidence that we have not seen.
"You need to satisfy yourself that that documentation is not relevant to this inquest. At this point you can’t say so as you haven’t seen the documentation."
Agreeing that the information should be made public to the court, Mr Travers announced a further pre-inquest review date for December 22 ahead of the inquest in January to allow all parties time to read the document.
The Government’s Cobra emergency committee was briefed by the Environment Agency on the day Zane died. A further meeting was held the following day chaired by David Cameron.
Last night former Minister Sir Eric Pickles, who chaired that Cobra meeting, told Mail on Sunday: ‘I do not recall hydrogen cyanide being mentioned at all. The briefing was wholly in the context of us being concerned about private generators.’
The documents also raise alarming questions for Surrey Police, which spent ten months investigating the pump theory.
The Mail on Sunday has previously revealed how council officials raised concerns about ground gases coming from the landfill site in 2007.
A spokeswoman from Spelthorne Council previously denied any wrongdoing on the council’s part in regard to the former landfill site.
She said: "Whilst we have the utmost sympathy for the family’s ongoing anguish, we have always maintained that there is no evidence of a link between the landfill and this tragedy, or of a public health risk.
A Major four-week full inquest touching the death of Zane will be held next year.
Related articles: Mail on Sunday