WikiLeaks co-founder Julian Assange missed a scheduled court appearance on Thursday because he is unwell, his lawyer said.
The 47-year-old had been expected to appear from prison via video link at a brief extradition hearing at Westminster Magistrates' Court, London.
He is in Belmarsh Prison serving a 50-week sentence for jumping bail in Britain while fighting extradition to the United States, which accuses him of violating the Espionage Act by publishing secret documents containing the names of confidential military and diplomatic sources.
Sweden also seeks him for questioning about an alleged rape, which Assange has denied.
It is not clear which claim would take precedence. The decision would likely be made by Britain's home secretary.
Lawyer Gareth Peirce told the court Assange was "not very well".
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.
Roughly two dozen supporters chanted "Free Assange" outside the court.
His case has attracted fresh support from free press advocates in the week since the US filed serious espionage charges against him.
He had earlier been held on suspicion of conspiracy to break into classified computer systems, a less serious charge.
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 health ward.
The group says Assange has "dramatically lost weight" and recently "it was not possible to conduct a normal conversation with him".
The complicated extradition process, which involves both Sweden and the United States and deals with press freedom and national security issues, is expected to last for months or years.
Assange was arrested by British police in April after Ecuadorian officials withdrew his asylum status. He had sought refuge in the Ecuadorian Embassy in 2012 when Sweden was trying to question him about sexual misconduct allegations.