In an open letter to the UK's Home Secretary Priti Patel, a team of 60 international doctors said the 48-year-old Australian should be moved out of the high-security Belmarsh prison in London to receive an assessment at a university hospital, Efe news reported.
Assange is facing an extradition process lodged by the US under the Espionage Act.
"It is our opinion that Assange requires an urgent expert medical assessment of both his physical and psychological state of health," the doctors said.
"Any medical treatment indicated should be administered in a properly equipped and expertly staffed university teaching hospital (tertiary care).
"Were such urgent assessment and treatment not to take place, we have real concerns, on the evidence currently available, that Assange could die in prison."
"The medical situation is thereby urgent. There is no time to lose."
Assange rose to international attention nine years ago after WikiLeaks published leaked military information provided by former US Army soldier Chelsea Manning, which prompted the United States government to launch a criminal investigation into the publisher.
In November that year, Sweden requested his extradition over allegations of sexual assault and rape.
He denied the allegations and also expressed concerns he would be extradited from Sweden to the US because of his role in the Manning leaks.
Assange surrendered himself to UK police around a month later and was held in custody for 10 days before being released on bail.
When he was unable to challenge the extradition proceedings, he breached bail and in August 2012 was granted asylum by Ecuador, taking refuge in the South American country's embassy building in London.
Sweden later withdrew its extradition request, but a UK arrest warrant remained in force as Assange had failed to surrender after breaking bail conditions.
The Swedish investigation was first opened in 2010 and closed in 2017, before being reopened earlier this year. It was subsequently closed again due to weakening evidence, although prosecutors said the plaintiff's claims were "credible."
This year, Ecuador lifted its asylum for Assange and the whistleblower was detained by UK police.
Assange faces charges including computer intrusion in the US.
The extradition request is being considered by UK judges.
( With inputs from IANS )