Wang Xuemei, a prominent Chinese forensic scientist, has cast doubt on the official version of the death of British businessman Neil Heywood, which triggered a huge political scandal in China.
Wang Xuemei said there was little evidence Neil Heywood died from cyanide poisoning.
Neil Heywood was found dead in a hotel room in November 2011.
Last month the wife of a prominent Chinese politician was found guilty of murdering him by poisoning.
However, the account given in court of how Gu Kailai killed Neil Heywood does not tally with cyanide poisoning, according to Wang Xuemei, who works for China’s top prosecutor’s office.
Cyanide poisoning would have caused lightning-fast asphyxia, spasms and a heart attack and turned his skin and blood bright red, which investigators would easily have spotted, she says.
A simple test for cyanide is also standard forensic practice in China, but none was presented in court, she adds.
Wang Xuemei says she believes Gu Kailai did have a motive to kill Neil Heywood and suggests that she used another poison to try and kill him.
No post-mortem examination was carried out on Neil Heywood’s body, which was cremated.
Gu Kailai was given a suspended death sentence, and her husband Bo Xilai, previously one of the most powerful figures in the ruling Communist Party, has been suspended from his position on the Politburo.
Gu Kailai’s aide, Zhang Xiaojun, was jailed for nine years for his part in the murder, while the regional police chief, Wang Lijun, received a sentence of 15 years for abuse of power and other offences.
The trials were closed to foreign journalists and no scientific evidence to show Neil Heywood was poisoned has been made public.
The new claims come just weeks before a crucial once-in-a-decade leadership change expected at a party congress this autumn.
From the very start there have been doubts about the official version of Neil Heywood’s death in a hotel room in Chongqing last November.
Initially the cause was said to be alcohol poisoning or a heart attack.
But in February, Wang Lijun fled the city after falling out with Bo Xilai and claimed that Neil Heywood had been murdered.
Bo Xilai’s supporters have claimed from the start that he is being framed by his political enemies.