The Wikileaks founder, 50, is said to have suffered a stroke at the time of a High Court battle over whether or not he should be extradited to the US.
He is being held at HMP Belmarsh, a high-security men’s prison in south-east London.
Stella Moris, 38, who is the mother of his two children, tweeted: “Julian Assange suffered a stroke on the first day of the High Court appeal hearing on October 27th. He needs to be freed. Now.”
In an interview with the Mail On Sunday, she said: “Julian is struggling and I fear this mini-stroke could be the precursor to a more major attack. It compounds our fears about his ability to survive, the longer this long legal battle goes on.
“It urgently needs to be resolved. Look at animals trapped in cages in a zoo. It cuts their life short. That’s what’s happening to Julian. The never-ending court cases are extremely stressful mentally.”
She added: “I believe this constant chess game, battle after battle, the extreme stress, is what caused Julian’s stroke on October 27… he was in a truly terrible state. His eyes were out of sync, his right eyelid would not close, his memory was blurry.”
Assange is wanted in America over an alleged conspiracy to obtain and disclose national defence information following WikiLeaks’ publication of hundreds of thousands of leaked documents relating to the Afghanistan and Iraq wars.
US authorities brought a High Court challenge against a January ruling by then-district judge Vanessa Baraitser that Assange should not be sent to the US, in which she cited a real and “oppressive” risk of suicide.
After a two-day hearing in October, the Lord Chief Justice Lord Burnett, sitting with Lord Justice Holroyde, ruled in favour of the US on Friday.
The senior judges found that the judge had based her decision on the risk of Assange being held in highly restrictive prison conditions if extradited.
However, the US authorities later gave assurances that Assange would not face those strictest measures either pre-trial or post-conviction unless he committed an act in the future that required them.
Ms Moris said on Friday that his lawyers intend to take his case to the Supreme Court, the UK’s highest court.
Justices will, however, have to decide first whether to hear the case before any appeal is heard.
A spokesman for the Ministry of Justice told the PA news agency it would not comment on individual cases.