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.
"Defence lawyer for Assange, Per Samuelson, said that Julian Assange's health state last Friday was such 'that it was not possible to conduct a normal conversation with him'."
The five-minute hearing on Thursday at Westminster Magistrates' Court was the second in the Australian's extradition case.
The US Justice Department has charged Assange with receiving and publishing thousands of classified documents linked to the wars in Iraq and Afghanistan.
The US wants the UK to extradite him, but Assange has formally refused consent.
On Thursday, chief magistrate Emma Arbuthnot set the next hearing for 12 June and suggested the hearing could take place in Belmarsh, adding: "It may be more convenient for everyone if it's there."
Supporters of Assange queued outside the court building for more than an hour to get into the public gallery.
Outside court, some chanted: "Free Julian Assange."
Assange sought political asylum in the Ecuadorian embassy in London in 2012. He is currently serving a 50-week sentence in Belmarsh Prison in south east London for bail violations.
Earlier this month, Swedish prosecutors reopened their investigation into rape allegations against Assange, which he denies.