WikiLeaks founder Julian Assange was unable to appear via video link at a US extradition hearing on Thursday due to illness, his lawyer said.
Assange, 47, who is currently serving a 50-week sentence in Belmarsh Prison for jumping bail in the UK, was due to appear at Westminster Magistrates Court.
"He's in fact far from well," Assange's lawyer, Gareth Peirce, told Reuters.
The US Justice Department seeks Assange’s extradition on 18 criminal counts related to actions they say violated the Espionage Act by publishing secret documents containing the names of confidential military and diplomatic sources.
WikiLeaks said in a statement it has "grave concerns" about Assange's health. The anti-secrecy group says he has been moved to the prison's medical ward.
The group says Assange has "dramatically lost weight" and recently "it was not possible to conduct a normal conversation with him".
Assange spent almost seven years holed up in cramped rooms at the Ecuadorian embassy in London, before being dragged from the premises by British police on April 18.
Authorities in Sweden are also seeking Assange’s extradition after reopening an inquiry into a rape allegation made against him. On May 13, Sweden's deputy director of public prosecutions, Eva-Marie Persson, said they would reopen the case into a report of alleged rape made to them on August 17, 2010.
She said there was still probable cause to suspect the crime occurred, although the investigation had been shelved while Assange hid in the embassy.
UK authorities will now need to decide which case takes precedence.
Judge Emma Arbuthnot said a more substantive extradition hearing set for June 12 may be moved to a court next to Belmarsh Prison for convenience.