The Foreign Office turned down a request from the Ecuadorian government to grant Julian Assange diplomatic status, amid reports the WikiLeaks founder may have received citizenship from the Andean country.
Mr Assange, 46, has been living in the London embassy of the south American country for five-and-a-half years after being granted political asylum as he fought sex-related claims in Sweden.
Quito said the country was seeking mediation to resolve the impasse. Following reports in Ecuadorean media that Mr Assange may have received citizenship, he posted a picture of himself on Twitter wearing an Ecuadorian football shirt on Wednesday.
An FCO spokesman said: "The Government of Ecuador recently requested diplomatic status for Mr Assange here in the UK.
"The UK did not grant that request, nor are we in talks with Ecuador on this matter.
"Ecuador knows that the way to resolve this issue is for Julian Assange to leave the embassy to face justice."
Swedish prosecutors have dropped their investigation into allegations against Mr Assange, but he still faces arrest for breaching bail conditions if he steps outside the embassy.
He also fears he will be extradited to the United States if he leaves the building and that there is a sealed indictment ordering his arrest.
How long has Assange been in the Ecuadorian embassy?
Reuters found an entry for "Julian Paul Assange" in Ecuador's Civil Registry, which only includes Ecuadorean citizens. A spokeswoman for the registry declined to comment when asked if Mr Assange had been granted citizenship.
Ecuador's foreign minister, Maria Fernanda Espinosa, said: "No solution will be achieved without international cooperation and the cooperation of the United Kingdom, which has also shown interest in seeking a way out."
A United Nations panel concluded in 2016 that Mr Assange was under arbitrary detention.
Profile | Julian Assange
A statement by Mr Assange's legal team said: "The UN ruling, issued almost two years ago, is crystal clear in its language. Mr Assange is unlawfully and arbitrarily detained by the UK authorities and must be released.
"The UK should not permit itself to be intimidated by the Trump administration's public threats to 'take down' Mr Assange."