WikiLeaks founder Julian Assange has missed a brief extradition hearing, apparently due to health problems.

Key points: Assange is currently in prison for skipping bail in Britain while fighting extradition to the US

Assange is currently in prison for skipping bail in Britain while fighting extradition to the US The judge says a more substantive extradition hearing is set for June 12

The judge says a more substantive extradition hearing is set for June 12 WikiLeaks releases a statement saying it has grave concerns about Assange's health

Scheduled to appear at the session from prison via video link on Thursday, Assange's lawyer Gareth Peirce said the Australian was "in fact far from well".

The United States requested the extradition of Assange, who was dragged from the Ecuadorean embassy in London on April 11.

Assange is in Belmarsh prison serving time for skipping bail in Britain while fighting extradition to the US.

Sweden also wants to question him about an alleged rape.

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.

WikiLeaks said it had grave concerns about Assange's health and that he had been moved to a health ward at the high-security prison.

"During the seven weeks in Belmarsh his health has continued to deteriorate and he has dramatically lost weight," it added in a statement.

"The decision of prison authorities to move him to the health ward speaks for itself."

'I am defenceless'

Swedish lawyer Per E Samuelsen said he visited the 47-year-old last week in London and Assange was so unwell it was not possible to have "a normal conversation with him".

Assange's lawyer, Gareth Peirce, said the Australian was "in fact far from well". ( Reuters: Hannah McKay )

WikiLeaks claimed Assange was "totally isolated and gagged".

Assange's supporters handed out copies of a letter they said was sent by Assange in prison in mid-May.

The ABC has not been able to verify its contents, but supporters say Assange has been writing to some contacts.

"I have been isolated from all ability to prepare to defend myself," the letter reads.

"I am defenceless.

"I am unbroken, albeit literally surrounded by murderers, but, the days where I could read and speak and organise to defend myself, my ideals, and my people are over until I am free."

Assange supporter Emmy Butlin protested outside the court. ( ABC News: Lincoln Rothall )

Emmy Butlin has been protesting against Assange's detention for several years and said he was clearly under strain.

"We are very concerned about the state of his health and we receive this [news] as a part of the persecution that this man has received all these years," she said.

"We appeal to the journalist profession and the publishing profession to stand by him."

WikiLeaks ambassador Joseph A Farrell said Assange's time in the Ecuadorian embassy was detrimental to his health. ( ABC News: Lincoln Rothall )

WikiLeaks ambassador Joseph A Farrell told the ABC the time Assange spent in the Ecuadorian embassy "was unsustainable" and detrimental to his health.

"He went into the prison in a worse state than somebody who might have gone into the prison straight off the street, plus being in a supermax prison is going to be hard on anyone," he said.

"The last year in the Ecuadorian embassy was particularly taxing.

"He had been cut off from all contact … the Moreno Government had started spying on him, which weighs on anybody."

Ecuador's Government has said Assange violated his asylum conditions and tried to use the Ecuadorian embassy in London as a "centre for spying".

Assange facing decades in prison

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 50 seconds 50 s Wikileaks founder Julian Assange is arrested at the Ecuadorian embassy in London.

The next hearing on the extradition request was set for June 12.

He faces a total of 18 US criminal counts with decades in prison if convicted.

Assange, who spent almost seven years holed up in cramped rooms at the embassy, has repeatedly said he fears extradition to the United States.

Earlier this month, he was sentenced to 50 weeks in prison by a British court for skipping bail after fleeing to the embassy.

The United States charged Assange with espionage, saying he unlawfully published the names of classified sources and conspired with and assisted ex-Army intelligence analyst Chelsea Manning in obtaining access to classified information.

Supporters gathered to protest Julian Assange's arrest. ( ABC News: Lincoln Rothall )

ABC/Reuters