Foreign Secretary Philip Hammond has branded a United Nations working group report on the "arbitrary detention" of WikiLeak founder Julian Assange as "frankly ridiculous".
Speaking at a joint press conference with his Iranian counterpart in London, Mr Hammond said Mr Assange was in fact "hiding from justice".
He spoke out after the UN working group ruled Mr Assange was being "arbitrarily detained" in the Ecuadorian embassy in London - and called for him to be paid compensation.
The UN Working Group on Arbitrary Detention said the Swedish and British authorities should end Mr Assange's "deprivation of liberty" and respect his physical integrity and freedom of movement.
Mr Assange is wanted for questioning over an alleged sex offence in Sweden but has avoided extradition by seeking refuge in the embassy, where he has been living for more than three years after being granted political asylum by the Ecuadorian government.
He claims he will be transported to the United States to be quizzed over the activities of WikiLeaks if he is extradited to Sweden. There is an espionage case against him in the US.
He filed a complaint against Sweden and the UK to the UN Working Group on Arbitrary Detention in September 2014.
Today Seong-Phil Hong, who heads the expert UN panel, said: "The Working Group on Arbitrary Detention considers that the various forms of deprivation of liberty to which Julian Assange has been subjected constitute a form of arbitrary detention.
"The Working Group maintains that the arbitrary detention of Mr Assange should be brought to an end, that his physical integrity and freedom of movement be respected, and that he should be entitled to an enforceable right to compensation."
But Mr Hammond said: "I reject the decision of this working group. It is a group made up of lay people and not lawyers.
"Julian Assange is a fugitive from justice. He is hiding from justice in the Ecuadorian embassy.
"He can come out any time he chooses... But he will have to face justice in Sweden if he chooses to do so.
"This is frankly a ridiculous finding by the working group and we reject it."
Mr Assange's legal team is due to give a news conference in central London later today and is expected to press for him to be allowed to leave the embassy without being arrested.
But the Swedish Prosecution Authority said the statement from the UN group had "no formal impact" on its ongoing investigation.
And the Metropolitan Police has said it will make "every effort" to arrest Mr Assange should he leave the embassy.