Chinese authorities have seized assets worth at least 90 billion yuan (£8.7bn) from family members and associates of retired domestic security tsar Zhou Yongkang, who is at the centre of China's biggest corruption scandal in more than six decades, two sources said.

More than 300 of Mr Zhou's relatives, political allies, proteges and staff have been taken into custody or questioned in the past four months, the sources, who have been briefed on the investigation, have said.

The sheer size of the asset seizures and the scale of the probes into people around Mr Zhou - both unreported until now - make the corruption probe unprecedented in modern China and would appear to show that President Xi Jinping is tackling graft at the highest levels.

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But it may also be driven partly by political payback after Mr Zhou angered leaders such as Mr Xi by opposing the ouster of former high-flying politician Bo Xilai, who was jailed for life in September for corruption and abuse of power.

Mr Zhou, 71, has been under virtual house arrest since authorities began formally investigating him late last year. He is the most senior Chinese politician to be ensnared in a corruption probe since the Communist Party swept to power in 1949.

"It's the ugliest in the history of the New China," said one of the sources, who has ties to the leadership.

The government has yet to make any official statement about Mr Zhou or the case against him.