BEIJING (Reuters) - Chinese lawyers taking on contentious human rights cases face limbo from Sunday after authorities did not approve an annual registration step the lawyers said was being used to stifle their work.

The dispute comes days before the 20th anniversary of the June 4 crackdown on pro-democracy protests in 1989. Rights lawyers are among the groups the ruling Communist Party has sought to contain at this sensitive time.

The 18 or so lawyers whose work may be stymied belong to a loose network of advocates who have challenged the government over deaths in prison and labor-reeducation camps, farmers stripped of their land, children sickened by toxic milk powder and other sensitive cases.

The annual registration required by government rules is usually a routine step that must be completed by May 31. But the lawyers facing effective disbarment said they appear to have been punished for taking on contentious clients, including members of the banned Falun Gong spiritual sect.

"All of the lawyers I know who haven't passed (the registration) are rights defense lawyers, so it looks like the authorities want to send a warning signal," said Jiang Tianyong, a Beijing-based lawyer who, among other causes, offered to represent Tibetans accused of crimes in riots in March last year.

He and six other lawyers interviewed said judicial officials have never expressly linked their registration problem to their rights advocacy.

"We think it may be because this year is so sensitive and we lawyers who don't follow orders are considered a threat," said Jiang. "But these things are never said so bluntly."

Xiao Lizhu, the official at the Beijing Bureau of Judicial Affairs in charge of registering lawyers, declined to comment on the rights lawyers. Li Bingru, secretary-general of the Beijing Lawyers Association, which helps administer the registration also refused to comment. "This is none of your business," he said.

Over the past decade, many rights activists have turned their energies to grassroots causes, challenging officials through court cases, public petitions and -- when censorship allows -- domestic media coverage.

Most of the lawyers interviewed said they were likely to receive the registration stamp eventually, but they were unsure whether they will be allowed to work until then.

"This seems to be a selective thing, more a warning than a permanent thing," said Zhang Kai, a rights lawyer who said he did receive the registration stamp after lobbying officials.

Lawyers who have not passed said they would continue working until told otherwise by officials.

Wang Yajun, preparing for a court hearing in northeast China on Monday about a member of Falun Gong, said he was unsure whether the court would accept his credentials.

"Our rights defense work ... will be threatened," said Li Xiongbing, another Beijing lawyer.

"If we've violated any laws, then we should be prosecuted. But this is just a covert form of punishment," he said.