BEIJING — China’s official news agency reported on Friday that Gao Zhisheng, a prominent human rights lawyer who has been missing for more than 20 months, will serve the next three years in prison because he repeatedly violated his probation.

The brief statement from a Beijing court provoked fresh outrage among human rights advocates who characterize Mr. Gao’s case as a leading example of abuse of dissidents in China. Although said to be free on probation for the past five years, Mr. Gao has been missing for much of the time since he was sentenced in 2006 for inciting subversion of state power.

His older brother, Gao Zhiyi, told the group China Human Rights Defenders that the court’s announcement proves that Mr. Gao has been in police custody even though the public security authorities have insisted that they had no knowledge of his whereabouts. Gao Zhiyi said he last saw his brother in April 2010 in the custody of public security officers; he is worried that his brother may be dead.

Renee Xia, one of the directors of the human rights group, said that the court’s declaration “is the clearest acknowledgement to date by the Chinese government that it has secretly detained Gao for the last 20 months despite its repeated denials.”