Update | China announces corruption probe into former security chief Zhou Yongkang

Beijing yesterday announced an official investigation into former security chief Zhou Yongkang, shattering the decades-old political taboo of not prosecuting the highest ranking Communist Party officials for corruption.

Zhou, who retired from the Politburo Standing Committee in 2012, is being probed for "serious disciplinary violations" - a euphemism for corruption. His case will be handled by the Communist Central Commission for Discipline Inspection, Xinhua said in a terse report.

The announcement ended intense speculation since the exclusively reported last August that current and retired party leaders had agreed to open a case against Zhou. He is the most senior Chinese official brought down by corruption in modern history and the case will have a far-reaching impact.

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It ends the unspoken rule agreed by party elders towards the end of the Cultural Revolution that Politburo Standing Committee members would not be prosecuted on the grounds of economic or social crimes - for the sake of the party's unity. It will also galvanise President Xi Jinping in his fight against corruption, which he has identified as the single biggest threat to China and the Communist Party.

"This is a big deal," said Professor Steve Tsang, director of the China Policy Institute at the University of Nottingham in Britain. "It breaks the convention that Politburo Standing Committee members are untouchable for criminal transgressions."

Other analysts and party sources said the announcement was intended to reduce behind-the-scenes struggles among top power holders and reunite the party under one banner ahead of a key gathering next month.

"The announcement suggests leaders have finally agreed on major policy issues, which will be finalised at the summer summit at Beidaihe [resort]," said Zhang Ming, a professor of political science at Renmin University. The Beidaihe summit is traditionally held in August.

On the same day, the leadership announced a key meeting for all party elite to be held in October to decide policy direction. President Xi said yesterday the rule of law would top the agenda at this meeting, the fourth plenary of the 18th Party Congress.