Ricardo Patino also said his government would announce on Thursday its decision on whether it would grant Assange's request for political asylum.
Patino told a news conference that Ecuador received from Britain on Wednesday "a written threat that it could assault our embassy" if Assange, who took refuge in the embassy on June 19, was not handed over.
Patinlo said the threat was delivered to Ecuador's Foreign Ministry in writing and verbally to the country's ambassador in London.
He said Ecuador "rejects in the most energetic terms the explicit threat of the official British communication," which he said Ecuador's ambassador in London, Ana Alban, received in a phone call.
President Rafael Correa said Monday that he hoped to announce a decision this week on Assange's asylum request and Patino said high-level consultations had occurred with Britain and Sweden.
The Australian, whose publishing via the Internet of thousands of sensitive U.S. documents including diplomatic cables and military dispatches, took refuge in the embassy to avoid extradition to Sweden, where he faces questioning for alleged sexual misconduct.
Assange's supports say the charges are trumped up and believe the U.S. has secretly indicted him and would extradite him from Sweden.