The 50-year-old is being held at the high-security Belmarsh Prison as he battles to avoid being extradited to America following a court ruling.
Assange has reportedly been left with a drooping right eyelid, memory problems, and signs of neurological damage following a mini-stroke.
It’s reported the stroke happened at the time of a High Court appearance via video link in October.
His fiancee Stella Moris said he is “struggling” with the stress of fighting extradition to a US prison.
Since the mini-stroke, Assange reportedly has had an MRI scan and is taking anti-stroke medication.
Ms. Moris told the Mail: “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.”
It comes after it was revealed Assange can be extradited to America to face espionage charges after the US Government won its bid to overturn a High Court ruling.
His lawyers have previously argued that he is a high suicide risk and is too ill to be sent to America for a trial.
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.
Mr. Assange faces an 18-count indictment from the US government, accusing him of conspiring to hack into US military databases to acquire sensitive secret information relating to the Afghanistan and Iraq wars, which was then published on the Wikileaks website.
If convicted in the US, Mr. Assange, 50, faces a possible penalty of up to 175 years in jail, his lawyers have said. However, the US government said the sentence was more likely to be between four and six years.