A British businessman who was killed in China had been providing information to the British secret service, the Wall Street Journal newspaper claims.
Neil Heywood had been communicating with an MI6 officer about top politician Bo Xilai for at least a year before he died, the paper said.
The UK Foreign Office said it would not comment "on intelligence matters".
In April, Foreign Secretary William Hague said Mr Heywood was not a government employee "in any capacity".
The case is at the heart of China's biggest political scandal in decades.
The November 2011 death of Mr Heywood brought down Mr Bo, the former Communist Party chief of Chongqing and a high-flier who was once tipped for top office.
Mr Bo's wife, Gu Kailai, was jailed in August for the murder of Mr Heywood at a Chongqing hotel. His former police chief, Wang Lijun, has also been jailed in connection with the scandal.
Mr Bo himself was expelled from parliament in September, stripping him of immunity from prosecution. He is accused of abuse of power, bribe-taking and violating party discipline, Chinese state media say, and is expected to go on trial in the future.
Ever since Mr Heywood's death plunged China into political crisis, there have been claims the Briton may have been a spy, says the BBC's Damian Grammaticas in Beijing.
Citing unnamed friends and British officials, the Wall Street Journal said that while Mr Heywood was not an MI6 employee, he had knowingly passed on information to the organisation.
"The Journal investigation, based on interviews with current and former British officials and close friends of the murdered Briton, found that a person Mr Heywood met in 2009 later acknowledged being an MI6 officer to him," the Wall Street Journal says in its report.
"Mr Heywood subsequently met that person regularly in China and continued to provide information on Mr Bo's private affairs."
Mr Heywood's relatives declined to comment, the paper added.
The British Foreign Office told BBC News it was "a longstanding policy that we don't comment on intelligence matters".
Whitehall sources have told the BBC that such contacts would have been informal - the type that might be expected in a small expat community - and have reiterated that he was never an agent of the intelligence service, says the BBC's Gordon Corera.
They say Mr Heywood was never paid and never formally tasked with passing on any information, our correspondent adds.
In a letter to a British MP on 26 April, Mr Hague addressed speculation over Mr Heywood, even as he said it was "long-established government policy neither to confirm nor deny speculation of this sort".
"However, given the intense interest in this case it is, exceptionally, appropriate... to confirm that Mr Heywood was not an employee of the British government in any capacity," he said.
The newspaper, citing unidentified sources, says this was technically true because Mr Heywood was not paid for his information.
But, says our correspondent, there are new questions about why, if Mr Heywood was known to Britain's intelligence services, British officials did not press their Chinese counterparts for a thorough investigation as soon as they knew he had died.
Mr Heywood, 41, had lived in China from the early 1990s, where he learned fluent Mandarin.
The nature of his association with Mr Bo and his wife Gu is not clear, but he has been described in some reports as a financial middleman. Chinese state media say Gu Kailai killed him over a business deal that went sour.
The case first came to light when police chief Wang Lijun fled to the US consulate in February, reportedly after falling out with Mr Bo over the Heywood case.
Chinese officials subsequently ordered that an investigation into Mr Heywood's death be reopened. Police had originally said he died of over-consumption of alcohol.
Five senior police officers in Chongqing have also been jailed, Chinese state media say, for covering up the case.