The Working Group on Arbitrary Detention found that Britain had not presented enough new information to merit a new examination.
The panel made the decision at a meeting last week, the UN human rights office said today.
In February, the panel found that Britain and Sweden had "arbitrarily detained" Assange, saying he should be freed and entitled to compensation.
Assange has been holed up in the Ecuadorean Embassy in London since 2012 to avoid arrest and extradition to Sweden. Swedish prosecutors want to question him over allegations of rape.
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