In an editorial, state-run agency Xinhua said the probe into Bo Xilai and his wife Gu Kailai over the death of Neil Heywood showed the Communist Party’s commitment to the rule of law.
The editorial came a day after Britain’s PM David Cameron raised the case with a visiting Chinese official.
It is one of several such pieces in state media in recent days.
Neil Heywood died on 15 November 2011 in the city of Chongqing, where Bo Xilai used to be the party chief.
Bo Xilai, now sacked from the post, is now being investigated for “serious discipline violations” and his wife Gu Kailai for “suspicion of homicide”.
The Xinhua editorial said the party had made “a resolute decision to thoroughly investigate related events and release information in a timely manner”.
The move showed its “determination to safeguard the socialist rule of law, to investigate and handle every discipline violation and never tolerate corruption”.
“The investigation into Bo… serves as a declaration to all party members that no matter what position one holds, party members shall never place themselves over party discipline and the law,” it concluded.
In recent days, a series of articles casting the downfall of Bo Xilai as a simple case of legal wrong doing have appeared in state media.
But the scandal – China’s biggest in years – comes ahead of a leadership change in Beijing due to get under way in October.
Some suspect that the party is using legal procedures as a pretext for a purge.
In political cases – such as this one – it is the Communist Party and not the courts that makes the decisions, our correspondent adds.
Britain, meanwhile, has called on China to carry out a full investigation free of interference.
PM David Cameron raised the case with Chinese politburo member Li Changchun on Tuesday when they met at Downing Street, offering “any necessary assistance” to help China investigate the death.
Chinese officials initially told British consular officials that Neil Heywood, 41, died of “excessive alcohol consumption”.
The UK government subsequently asked the Chinese authorities to reopen the investigation. Unconfirmed reports in China suggest Neil Heywood may have died from cyanide poisoning.
State media have reported that Gu Kailai and Zhang Xiaojun, an orderly at Bo Xilai’s home, have been arrested.
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