Speaking with Swedish Radio News, Hammond says the findings of the United Nation's Working Group on Arbitrary Detention is not in any way approved by the UN at large or its members.
The Foreign Secretary also rejected the panel's claim that Assange is imprisoned and points out that he can leave the embassy whenever he wants.
"We don't accept the conclusions of the working group," he said. "Mr. Assange is not detained in any way."
On Friday, Assange touted the decision from the balcony of the embassy as clear proof of his mistreatment by both the Swedish and British government.
Assange has been wanted for questioning since 2010 for alleged sex crimes against two women. After refusing to be questioned in Sweden, Assange has sought refuge in the Ecuadorian embassy since 2012.
Hammond said Assange was "hiding from justice" and called on him to leave the embassy, adding that "we have every confidence that he will get a fair hearing in Sweden."
Swedish authorities reject UN panel's Assange findings0:44 min 0:44 min
UN panel: Assange arbitrarily detained by Sweden, UK10 min 10 min