Julian Assange was too ill to appear by video link for the latest hearing in relation to his possible extradition to the US, lawyers for the WikiLeaks founder told a court.

The hearing was the first since 2 May, when lawyers for the US government began pressing its case to extradite him to face trial for what they described as one the largest compromises of classified information in history.

However, his solicitor Gareth Peirce told a judge at Westminster magistrates court in London on Thursday that Assange was too ill to appear by video link from prison. The date for the next hearing was confirmed as 12 June.

The senior district judge Emma Arbuthnot said the next hearing may take place in Belmarsh prison in south London, where Assange is being held, if convenient for all parties.

The US has requested the extradition of Assange, who was removed from the Ecuadorian embassy in London on 11 April. He is charged with conspiracy to commit computer intrusion, which carries a maximum penalty of five years.

But he could face decades in a US prison after it emerged last week he was also being charged with violating the Espionage Act by publishing classified information through WikiLeaks.

Prosecutors announced 17 additional charges against Assange for publishing hundreds of thousands of secret diplomatic cables and files on the wars in Afghanistan and Iraq.

Assange, 47, was previously charged with working to hack a Pentagon computer system, in a secret indictment that was unveiled soon after his arrest at Ecuador’s embassy.

WikiLeaks said on Tuesday the Australian had been moved to the hospital wing of Belmarsh after a “dramatic” loss of weight and deteriorating health.

“Mr Assange’s health had already significantly deteriorated after seven years inside the Ecuadorian embassy, under conditions that were incompatible with basic human rights,” WikiLeaks said in statement.

“During the seven weeks in Belmarsh his health has continued to deteriorate and he has dramatically lost weight. The decision of prison authorities to move him to the ward speaks for itself.”

Assange is serving a 50-week prison term for breaching bail conditions to avoid extradition to Sweden. He took refuge in the embassy in in 2012 to avoid extradition over sexual assault allegations, which he denies, but was arrested last month after his asylum was revoked.