Britain has refused Ecuador's request to give Julian Assange safe passage for a medical checkup after he had a sharp pain in his right shoulder, Quito's top diplomat said.
The WikiLeaks founder has been holed up in the Ecuadoran embassy in London since 2012, seeking to avoid extradition to Sweden.
Swedish prosecutors want to question Assange about a rape claim, which carries a 10-year statute of limitations that expires in 2020.
Assange, who faces arrest if he tries to leave the embassy, denies the allegation and insists the sexual encounter was consensual.
"We did ask the British government for a safe passage for humanitarian reasons in coordination with Ecuador, so that Julian Assange can get an MRI," Foreign Minister Ricardo Patino told a briefing in Quito.
"The reply we have had from Britain is that he can leave whenever he likes for any medical care he might need but the European arrest warrant for Assange is still valid. In other words, he can leave -- and we will put him in jail," Patino added.
Patino spoke two days after Britain said it would stop standing guard non-stop outside the Ecuadoran embassy in London.
The 44-year-old Australian also fears that if he leaves he could eventually face extradition to the United States and a trial over the leak of hundreds of thousands of classified military and diplomatic documents in 2010.
Patino earlier said that "the British police have no reason to spend so much money to deploy so many police and vehicles outside the embassy."
He recalled that Sweden and Ecuador are negotiating what he called an international criminal assistance accord.
Sweden wants this accord finished by the end of the year so it can question Assange inside the Ecuadoran embassy in London.
Patino said no deadline for the agreement has been set.