The anti-secrecy group's chief has been based at the Ecuador embassy since 2012 after he lost appeals against sexual assault charges in Sweden.
Swedish prosecutors dropped the rape investigation in May previous year, but Assange still faces arrest by British police on a charge of skipping bail if he leaves.
According to the AP, the British government said Thursday that it would not grant diplomatic status to Assange. Assange also fears a possible US extradition request stemming from the leaking of classified USA documents. He's been holed up in the embassy for over five years now, with little hope of being able leave a free man.
Following the announcement from the Foreign Office, Assange in a tweet said, "The US & UK governments continue their attempts to arrest me in violation of two United Nations rulings and their own laws".
His stay in the Ecuador embassy has been reported to have cost the Metropolitan Police in London several million USA dollars in police presence outside the embassy. According to a MET Police statement from last May, that's because an arrest warrant was filed after Assange failed to surrender himself to court on 29th June 2012. It would have enabled him to walk out of the embassy without fear of arrest. She said in a news conference Thursday in Quito, the Ecuadoran capital, that Assange was granted citizenship on December 12, after having applied for it in September.
Ecuador said the United Kingdom should guarantee him "safe passage" to Ecuador if he leaves the embassy. The Swedish prosecutors halted the case against Assange in September 2017.
He said the decision to grant him asylum in 2012 had "generated problems" in Ecuador and had only discredited the country overseas, particularly in the United States.
The move was seen as an unsuccessful attempt to allow Assange to leave the embassy without facing arrest by British police.
The UK's rejection of a diplomatic pass comes after Assange tweeted a cryptic picture of him dressed in the colours of Ecuador.