Labour has demanded an inquiry into what killed a seven-year-old boy during flooding in Chertsey on what would have been his 14th birthday.
Zane Gbangbola's parents claim hydrogen cyanide gas from a former dump behind their home killed their son in 2014.
They have said an inquest that ruled he died from carbon monoxide was flawed.
Shadow Justice Secretary David Lammy has told three secretaries of state further evidence "must be considered". The government said it would respond.
An online anniversary event for Zane was held on Wednesday evening.
Writing to Justice Secretary Robert Buckland, Home Secretary Priti Patel and Environment Secretary George Eustice, Mr Lammy said there were doubts over toxicology results and hydrogen cyanide readings.
He said experts from Porton Down attended the site on 8 February, however "their involvement was withheld from the inquest".
Mr Lammy said claims of chemical dumping in the area, revealed by the BBC, were among many issues "not considered".
He said concerns were "recognised across the political spectrum", with Conservative-led Spelthorne council also demanding an inquiry.
The government has not yet responded to Spelthorne council's request.
"Not only is this vital to bringing closure for Zane Gbangbola's family," Mr Lammy said, "but also in providing public confidence in the ability of our institutions to uncover the truth."
In 2016, Surrey Coroner Richard Travers ruled Zane died from carbon monoxide poisoning from a petrol-powered pump used to clear floodwater from the house on the banks of the Thames.
Zane's parents, Kye Gbangbola and Nicole Lawler, have always insisted the pump was not in use.
Ms Lawler recalled how the couple had to deal with the authorities as they faced the loss of their son.
She said: "We were at our most vulnerable in our lives. We had lost our child.
"We expected that those in authorities would be there to shield us to protect us too, to be as motivated as we were to find out what had happened and how this happened.
"What we experienced were public authorities running around like headless chickens trying to cover their tracks rather than doing the right thing, and I can't explain in words how that felt."
She said: "Zane meant the world to us and the pain that we feel daily is relentless.
"It's as raw today as it was the day that Zane died and it's the kind of tears that fall from your soul.
"I just wish I could have given my life to save him. What I can do is I can devote my life to getting justice for him and to getting the truth and to make sure that this never happens again."
A spokeswoman for the Coroner's Service said: "An inquest into the death of Zane Gbangbola was concluded in September 2016. The inquest ran for over six weeks, during which time the coroner heard a substantial amount of evidence including that from in excess of 70 witnesses.
"Having considered all the evidence that was before the court, the coroner concluded that the cause of Zane's death was carbon monoxide toxicity.
"We would like to offer our sincere condolences to Zane's family and friends on his tragic death."
A government spokeswoman said: "This is a tragic case and our thoughts remain with the Gbangbola family.
"We will respond to the letter [from David Lammy] in due course."