Julian Assange is receiving a visit from his father and artist Ai Weiwei at Belmarsh prison.
The WikiLeaks founder was moved to a medical ward in the high security jail last month and his supporters expressed “grave concerns” about his health.
Assange is serving a 50-week prison sentence after being dragged out of the Ecuadorian embassy in April and jailed for a bail violation.
The 47-year-old’s father John Shipton arrived at the prison with Chinese activist Ai Weiwei on Tuesday afternoon.
Earlier this month, a Swedish court rejected a request from prosecutors for the Australian to be detained in his absence.
Prosecutors had said they would issue a European Arrest Warrant if the request had been granted, but Uppsala District Court rejected the request.
An investigation has been reopened into an allegation of rape in Sweden, which Assange has always denied.
He also faces an extradition request from the US on allegations of spying.
A case management hearing for that matter is due to take place on Friday at Westminster Magistrates’ Court, and Assange is expected to appear via video link from prison.
He did not appear at a short hearing on May 30, where chief magistrate Emma Arbuthnot and solicitor Gareth Peirce referred to him as “not very well”.
Just a few hours before that hearing, WikiLeaks said it had “grave concerns about the state of health of our publisher, Julian Assange, who has been moved to the health ward of Belmarsh prison”.
A spokesman added: “During the seven weeks in Belmarsh his health has continued to deteriorate and he has dramatically lost weight. The decision of the prison authorities to move him into the health ward speaks for itself.”
Assange sought political asylum in the Ecuadorian embassy in London in 2012 after the leaks of hundreds of thousands of classified US diplomatic cables on his whistleblowing website.
He was charged in the US last month with receiving and publishing thousands of classified documents linked to the wars in Iraq and Afghanistan.
The US justice department indicted Assange on 18 counts that relate to his “alleged role in one of the largest compromises of classified information in the history of the United States”, it said.
He is accused of working with former US army intelligence analyst Chelsea Manning in “unlawfully obtaining and disclosing classified documents related to the national defence”, a statement said.