China's government sacked an ambitious senior official, Bo Xilai, after his attempt to block a criminal investigation involving his family, according to two former officials.

Bo's abrupt dismissal as Communist Party chief of Chongqing municipality in southwest China last week exposed tension before a national leadership transition later this year.

The two ex-officials, who spoke to Reuters on condition of anonymity, said that on the weekend the central leadership circulated an account of the tension between Bo and his once trusted aide, Wang Lijun, a vice major and until recently Chongqing police chief.

The document for the first time confirms that differences between Bo and Wang over the criminal investigation led to Wang seeking asylum at the U.S. consulate in Chengdu, where he fled on February 6.

He left the consulate with central Chinese officials a day later. The U.S. State Department has said Wang left the voluntarily.

The two former officials said the internal account, apparently based on Wang's testimony and delivered to closed-door meetings in Chongqing and elsewhere, left scant doubt that Bo's hopes for a spot in the next central leadership were over, and he could be implicated in accusations of abusing power.

The central leadership has said that Bo bears leading responsibility for the Wang Lijun incident, said one of the former officials with close links to central leaders, citing sources in Chongqing.

He said Wang's criminal investigation involved allegations about Bo's wife, Gu Kailai, but he had no details. Soon after the confrontation, Bo moved Wang from his public security portfolio to one covering education, compounding Wang's fears that he would be purged, said the ex-official.

The case had been under investigation for some time, said the second ex-official. It seems Wang was using it to ensure Bo Xilai protected him, but instead Bo Xilai turned on him and Wang began to fear he would be trapped.

AMBITIOUS, UNSCRUPULOUS

The allegations offer a glimpse into Bo's ascent to prominence in Chongqing, a haze-shrouded city-province beside the Yangtze River where he was sent in 2007 after what observers said was a failed effort to rise from commerce minister to vice premier.

Even without the Wang Lijun incident, many people were opposed to Bo joining the Standing Committee, said the second former official. He was ambitious and unscrupulous, and that goes against how Chinese leaders are supposed to appear.

Both ex-officials spoke on condition of anonymity citing government sensitivity about the case. China's State Council Information Office did not reply to faxed questions about the case.

After arriving in Chongqing in 2007, Bo, 62, turned it into a bastion of Communist red culture and egalitarian growth, also winning national attention with a crackdown on organised crime that was spearheaded by Wang Lijun.

Bo's self-promotion and revival of Mao Zedong-inspired propaganda irked moderate officials. But his populist ways and crime clean-up were welcomed by many residents and others who hoped Bo could try his policies nationwide.

Bo himself has disappeared from public view. He is being kept in Beijing while the investigation into Wang continues, said the two ex-officials.

The speculation about Bo's future rippled through China's annual parliament session that ended last week. Bo used a news briefing at the session to hit back at unidentified foes.

He suggested that people spreading accusations about his son and wife were serving the agenda of crime bosses that he fought in Chongqing.

These people who have formed criminal blocs have wide social ties and the ability to shape opinion, he said of his critics.

There are also, for example, people who have poured filth on Chongqing, and poured filth on myself and my family.

(Editing by Michael Urquhart and Robert Birsel)