BEIJING (Reuters) - A prominent Chinese rights lawyer who wrote an open letter criticizing President Xi Jinping said on Monday that authorities had revoked his license to practice law.

FILE PHOTO: Chinese President Xi Jinping speaks during a joint press briefing with French President Emmanuel Macron, not shown, at the Great Hall of the People in Beijing, China January 9, 2018. REUTERS/Mark Schiefelbei/Pool

Beijing-based lawyer and activist Yu Wensheng has repeatedly criticized the ruling Communist Party over a multi-year sweeping crackdown on rights lawyers and activists, which has seen hundreds detained and dozens arrested.

Chinese authorities briefly detained Yu in October after he wrote an open letter saying Xi was unsuited to lead China as he had strengthened “totalitarian” rule over the country.

Yu told Reuters on Monday that he believed the revoking of his legal license was part of reprisals for penning the open letter, but that it only made him more determined to persevere with his rights activism.

“It won’t slow me down,” he said.

The Beijing Municipal Bureau of Justice told Yu in a letter that his license had been canceled because he had not been employed by a registered law firm for over six months, according to a picture of the letter seen by Reuters.

A man who answered the telephone at the bureau said he was unfamiliar with the case and declined to make further enquiries.

A set of measures on the practice of Chinese law firms and lawyers was revised in late 2016 to include higher levels of scrutiny of the speech and conduct of lawyer, as well as greater requirements of political loyalty for firms and practitioners.

Rights groups say the revisions were aimed at preventing the legal community from taking on sensitive rights cases or speaking out against political prosecution of legal work.

Yu had also been involved in attempting to defend a fellow rights lawyer Wang Quanzhang, who was detained during a spate of lawyer detentions in July 2015. Authorities have blocked his attempts to meet Wang.

Wang, who has been detained for over 900 days, has been charged with subversion of state power. Wang’s wife, who has protested on her husband’s behalf, had asked Yu to defend him after rejecting court-appointed lawyers.

Yu’s wife Xu Yan said in a statement sent to Reuters that she and Yu had spent over 150,000 yuan attempting to set up an independent legal practice for Yu after he left his old firm last year.

“Now, the Beijing justice bureau says its not going to happen, so it’s not going to happen,” she said.