BEIJING (Reuters) - China's disgraced former domestic security chief Zhou Yongkang looks set to be expelled from the ruling Communist Party at a key meeting next week, sources said, possibly paving the way for his formal prosecution.

Mr Zhou is the most senior party member to have been targeted in a corruption probe since the Communists swept to power in 1949, and President Xi Jinping has made fighting graft a cornerstone of his administration.

At a four-day party plenum beginning on Monday in Beijing, the Central Commission for Discipline Inspection, its main anti-graft body, will present its findings in a report on Zhou, said three sources, who all spoke on condition of anonymity. "The Central Committee is expected to approve (a proposal to) expel Zhou for grave violations of party discipline and decide whether to turn him over for prosecution," one source with leadership ties told Reuters.

The source added that the judicial process could be lengthy. "If the Central Committee votes to prosecute Zhou, the process will drag out due to a lack of (judicial) manpower and the complexity of the case."

The Central Committee is the largest of the party's elite decision-making bodies, made up of some 200 members who can vote and around 170 alternate members who do not have a vote.

The party announced in July that Mr Zhou was being investigated by its anti-corruption watchdog for suspected "serious disciplinary violations", the usual euphemism for graft, although it could also imply other wrongdoing.

The move followed months of speculation about his fate.

It has not been possible to reach Mr Zhou for comment on any of the allegations against him.

Mr Zhou, 71, was the security tsar within the Politburo Standing Committee, China's apex of power, for five years until he retired in 2012.

For Mr Xi, taking on such a senior figure appears to be a calculated risk. The public move against Mr Zhou is the pinnacle of a campaign to bring down "tigers" and "flies", or corrupt officials of senior and low rank.

By breaking an unwritten rule that members of the Standing Committee would not come under scrutiny after retirement, Mr Xi could antagonise other party elders who fear that they and their families could be next if the crackdown does not ease off after Mr Zhou's investigation, political analysts said.

However, sources told Reuters that two influential former Chinese leaders Hu Jintao and Jiang Zemin gave their consent for Mr Xi to investigate Mr Zhou.

Also, unlike Bo Xilai, a charismatic ally of Mr Zhou's who was felled from his position as Chongqing party chief in 2012 in another corruption scandal, Mr Zhou has little public sympathy, and many of his supporters in the party have already been removed.

Expulsion from the party is the usual precursor to handing over corruption cases to the judiciary for prosecution.

Rule of law has for the first time been made the main topic for discussion at the plenum, a meeting held most years and always amid tight security and great secrecy. Previous plenums have discussed the fate of fallen senior officials, however. "He will almost certainly be expelled from the party at the plenum. His case will definitely come up, as this is what has happened in past plenums," said Mr Zhang Ming, an expert on domestic politics at Beijing's Renmin University.

Mr Zhang added that a slight element of doubt existed about whether the expulsion could happen next week because Mr Zhou is a retired official and does not hold any posts. "The law is the yardstick for political life in China which nobody can transgress, including the highest levels of leadership," the party's official People's Daily wrote on its website earlier this month. "For somebody of the level of Zhou Yongkang, the investigation into his discipline violations should reach a conclusion at the fourth plenum," it added, using the formal name for next week's meeting.

Mr Zhou has been put under virtual house arrest since late 2013 and investigated for corruption involving family members and political allies as well as ordering the bugging of the telephones of top leaders and the mysterious death of his former wife in a road accident, sources have said.

More than 300 of Mr Zhou's relatives, political allies and their business associates have been arrested, detained or questioned over the past two years, according to sources briefed on the investigations.

The trials of some of Mr Zhou's allies could start as early as this year, the sources said.

Chinese authorities have seized assets worth at least 90 billion yuan from these people, the sources added.

But the party is unlikely to reveal the entire story because it would further undermine its image and make it difficult to explain to the public how Mr Zhou climbed the political ladder to eventually become a member of the Politburo Standing Committee, sources and analysts said.

Once China's top law enforcement officer, Mr Zhou has now become the villain of choice for an anti-corruption campaign that Xi says is essential to bolster the party's legitimacy and perpetuate its rule.

Ironically, much of Mr Zhou's career was devoted to this very aim - preserving the party's monopoly on power.

Mr Zhou cut his teeth in the oil industry and made the switch to police work as minister of public security between 2002 and 2007.

He then became secretary of the party's Politics and Law Commission, giving him sway over China's judges, prosecutors, the police, the paramilitary People's Armed Police and the civilian intelligence agency.

The security apparatus he ran was ruthless in crushing dissent, expanded during his watch and consumed a budget that exceeded the official figure for military spending.