London (CNN) Julian Assange was too ill to appear via video link at a court hearing in London on Thursday, his lawyer said, after WikiLeaks expressed "grave concerns" about the health of its founder and said he had been moved to the medical ward of his prison.

Assange had been expected to participate in the hearing at Westminster Magistrates' Court via video link, but was instead represented solely by his solicitor, who told the chief magistrate his client was "not very well."

The hearing, on an extradition request from the United States over a computer hacking conspiracy charge, was adjourned until June 12 and could next take place in Belmarsh prison, where the publisher is being detained.

"It may be more convenient for everyone if it's there," chief magistrate Emma Arbuthnot said.

Earlier on Thursday, WikiLeaks released a statement on Assange's wellbeing. "During the seven weeks in Belmarsh his health has continued to deteriorate and he has dramatically lost weight," the statement read. "The decision of prison authorities to move him to the health ward speaks for itself."