LONDON — The mental and physical condition of Julian Assange has so deteriorated that he could die in a British jail before his February hearing on extradition to the United States, a group of international doctors has warned.

In an open letter to Britain’s home secretary, Priti Patel, more than 60 doctors called for Mr. Assange, the 48-year-old founder of WikiLeaks, to be transferred from the high-security Belmarsh prison in London to a university teaching hospital to receive an expert medical assessment.

“Were such urgent assessment and treatment not to take place, we have real concerns, on the evidence currently available, that Mr. Assange could die in prison,” the letter said. “The medical situation is thereby urgent. There is no time to lose.”

Their assessment is based on witness accounts from an October hearing at Westminster Magistrates’ Court, in which Mr. Assange was described as “exhibiting the symptoms of a torture victim.” The analysis was corroborated this month by Nils Melzer, the United Nations special rapporteur on torture, who wrote a report about Mr. Assange’s health, warning that his life was at risk.