Spelthorne council had asked for a full inquiry into Zane Gbangbola's death.
Zane's parents say he was killed by hydrogen cyanide gas washed out of a former dump behind their home.
An inquest ruled he died from carbon monoxide from a petrol-powered pump used to clear floodwater in the house.
Kye Gbangbola and Nicole Lawler have said the pump was not in use.
Department for Environment Food & Rural Affairs (Defra) minister Rebecca Pow replied to Spelthorne council's request and wrote: "If Zane's parents believe that there is new evidence available, they should consider making an application to the Attorney General asking her to apply to the High Court to quash the inquest and order a fresh coroner investigation."
Shadow Justice Secretary David Lammy said: "Telling Zane's parents it is up to them to take yet more action, after all they have sacrificed in the seven years since their son's tragic death, is a gross insult.
"The government should take responsibility for finally allowing the truth to be uncovered.
"There is crucial and compelling evidence, which was unavailable to the coroner, that must now be considered by experts in an independent panel inquiry."
Spelthorne councillor Ian Beardsmore, who proposed the council's action, said he was "deeply disappointed".
In a statement, Zane's parents said: "It is simply wrong to deny a child killed in these circumstances a proper investigation.
"This is an effort to continue to kick Zane's case into the long grass. Justice delayed is justice denied."
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