A top Chinese scientists has raised questions over whether Neil Heywood was really poisoned, writing in a blog post that there is "little evidence" cyanide is what killed the British businessman.
Heywood was found dead in a Chongqing hotel room in November 2011. His death caused a huge political uproar in China when Gu Kailai, the wife of prominent Chinese Communist Party politician Bo Xilai, was accused and found guilty of poisoning him.
Wang Xuemei, a forensic scientist in the Supreme People's Procuratorate, said: "I feel very pained, upset and scared that our court believed the theory [Heywood] was poisoned with cyanide."
Wang said cyanide poisoning would have caused immediate asphyxia and a heart attack, turning his skin and blood bright red, but no such detail, or a forensic test for cyanide, was ever presented in court.
"A serious lack of evidence exists yo conclude that Neil Heywood died of cyanide poisoning, as well as any supporting scientific basis."
"Any forensic doctor in China would have immediately spotted something like that out of the ordinary."
Gu was given a suspended death sentence, with her aide, Zhang Xiaojun, jailed for nine years for his part in the murder.
Wang Xuemei's comments are some of the few Chinese views that have filtered out to Western media regarding the case.
There has been heavy censorship of discussion of the case in Chinese media and on social networks.
In her blog post Wang Xuemei also suggested that the former police chief Wang Lijun, who received a sentence of 15 years for abuse of power and other offences revolving around the murder, could have manipulated Gu.
She wrote in the blog: 'In other words, Wang Lijun could easily have used Gu to do whatever he wanted to do.
"I just want to tell people I feel humiliated. I think Chinese criminal doctors are not such idiots. I have done my duty and fulfilled my historical responsibility."