Mr Yang is being investigated by China’s state security services for allegedly endangering national security and had been held in a secret location with a monthly consular visit from the Australian embassy but no access to family or lawyers. Six months is the longest a suspect can be held under this process.

“His residential surveillance period has expired now and the compulsory measures against him now become criminal detention. His family has been informed to pick up the [detention] notice this afternoon,” said Mr Mo.

The maximum period of criminal detention is 37 days. After it expires, the prosecutors will decide whether to formally arrest him or not. He could be released after this time on bail pending a trial.

Mr Stary said his client had been moved to the Beijing's Public Security Bureau after six months under so-called "residential surveillance at a designated location".

"We know that the form of his detention has changed and he has moved from what might be said to be a conventional detention facility to a facility that is used for internal security suspects," he said.