BEIJING (REUTERS) - China has charged former domestic security chief Zhou Yongkang with leaking state secrets because he warned political ally Bo Xilai in 2012 that Bo was about to be ousted, a crime that can be punishable by death, sources told Reuters.

Although prosecutors charged Zhou on April 3 with intentional disclosure of state secrets, the link to Bo has not previously been made public. Bo, a charismatic politician and one-time rising star within the ruling Communist Party, was jailed for corruption and abuse of power in 2013.

Sources with ties to the Chinese leadership said Zhou, 72, stood accused of tipping off Bo that he was about to be sacked in early 2012 as party boss of the south-western metropolis of Chongqing and a member of the decision-making politburo. "Zhou Yongkang told Bo Xilai about the central leadership's decision to bring him down," one of the sources said, requesting anonymity due to the sensitivity of the case.

Neither the state prosecutor's office, the cabinet information office nor the party's graft watchdog, the Central Commission for Discipline Inspection, responded to requests for comment.

Prosecutors also charged Zhou with bribery and abuse of power, although they have given no details about one of the most dramatic scandals to hit China since the Communist revolution in 1949.

No date has been set for Zhou's trial, although the sources said authorities would likely appoint a lawyer for him. Zhou has not been seen in public since October 2013. It has not been possible for Reuters to reach him for comment.

Before Bo's downfall, Zhou had recommended that Bo succeed him as domestic security chief on the party's Politburo Standing Committee - the apex of power in China - sources with direct knowledge of the matter have previously told Reuters.

They have also said President Xi Jinping was determined to bring down Zhou for plotting such appointments to try to retain influence after the party's 18th Congress in November 2012, when Xi took over the party and Zhou retired from the standing committee. "With knowledge of his impending ouster, Bo could have absconded abroad or sought political asylum at an embassy," a second source with leadership ties said, adding this would have complicated the case against Bo.

The sources said they did not know if Bo had contemplated fleeing China. They also did not say how or why Zhou tipped off Bo, who could have already been under surveillance.

Bo disappeared from public view around the same time he was sacked from his senior party positions.

Li Guifang, a lawyer who represented Bo during his trial, declined to comment when reached by Reuters and asked about the connection between the two men.

Bo's career nosedived after his wife Gu Kailai was implicated in the 2011 murder of a British businessman, Neil Heywood, who had been a family friend. She was given a suspended death sentence in 2012.

That scandal came to light after Bo's estranged police chief, Wang Lijun, unsuccessfully sought political asylum at the U.S. consulate in the southwestern city of Chengdu in early 2012.

Wang said at his trial that he fell out with Bo over what to do about the murder. Wang was jailed for trying to cover up the killing, for defection, abuse of power and bribe-taking.

Under Chinese law, Zhou's crimes are punishable by death, sources and legal experts said.

Zhou could be sentenced to death, but it might be suspended because of his age and if he showed remorse, said the first source with leadership ties.

The party, which controls the courts, usually determines verdicts in high-profile trials ahead of time, experts say.

Zhou will face trial in the northern port city of Tianjin, near Beijing.

A source close to the military and the first source with leadership ties said Zhou was being guarded in Tianjin by People's Liberation Army soldiers instead of police in case elements of the civilian force were still loyal to him.

As security tsar, Zhou oversaw the police, the civilian intelligence apparatus, judges, prosecutors and paramilitary police. The senior leadership deemed his portfolio too powerful and downgraded it at the 18th Congress, sources have said.

Authorities have seized assets worth at least 90 billion yuan (S$19 billion) from Zhou's family members and associates, sources have said. "The wealth his clique amassed rivalled that of (some developing) countries," the second source with leadership ties said.

Zhou's alleged crimes took place over decades, including when he was vice president of state-owned China National Petroleum Corporation (CNPC), party boss of Sichuan province, minister of public security and a member of the standing committee, prosecutors said this month.

CNPC has publicly backed the government's decision to investigate Zhou. A company spokesman said he had nothing further to add.