WikiLeaks founder Julian Assange is reported to be suffering from a chronic lung ailment.
Ecuador's ambassador to Britain Ana Alban said his condition could worsen at any time and he is being checked regularly by doctors.
Mr Assange, 41, whose website angered the US by releasing thousands of secret diplomatic cables, has been inside Ecuador's embassy in London since June.
He is avoiding extradition to Sweden for questioning over rape and sexual assault allegations.
Mr Assange has denied any wrongdoing.
"In regard to how he is doing, what I can say is, imagine how well someone is in a space of 50 square metres without much sun and poor air circulation," Ms Alban said.
British authorities say he will be arrested if he sets foot outside the embassy.
The building, located just behind London's Harrods department store, is under constant police surveillance.
Ecuador said last month it is worried about Mr Assange's health and asked Britain to guarantee him safe passage to hospital from the embassy if he needs medical treatment.
Mr Assange took refuge in the embassy after running out of legal options to avoid being sent to Sweden.
Ecuador granted him asylum in August and said it shared his fears that he could be sent from Sweden to the US to face charges over WikiLeaks' activities.
US and European government sources say the United States has issued no criminal charges against him, nor launched any attempts to extradite Mr Assange.