Julian Assange was too ill to appear at his court hearing in London on Thursday, his lawyer said.
The WikiLeaks founder, 47, is fighting against being extradited to the United States over charges related to leaking US government secrets.
He had been due to appear at his case management hearing via video link from Belmarsh Prison but lawyer Gareth Peirce said he was “not very well”.
According to WikiLeaks, Assange has been moved to the medical ward in jail.
A spokesman for the whistleblowing website said it had “grave concerns” about Assange’s health.
“During the seven weeks in Belmarsh his health has continued to deteriorate and he has dramatically lost weight,” the spokesman said.