In a court in the British capital, Judge Emma Arbuthnot rejected his lawyers' argument that upholding the warrant was no longer in the public interest.
"I find arrest is a proportionate response even though Mr has restricted his own freedom for a number of years," she said.
Assange was awaiting extradition to Sweden over a 2010 investigation into claims of rape and sexual assault when he breached his bail conditions to seek refuge in the embassy.
Sweden dropped its investigation last year, but the British warrant for his arrest for jumping bail remains, meaning he would be detained as soon as he steps outside.
The judge rejected claims that Assange was suffering from his confinement, saying: "Mr Assange's health problems could be much worse."
She had last week rejected claims by Assange's team that the warrant was rendered null-and-void because there was no longer any underlying crime.
Assange says he fears his arrest could lead to his extradition to the United States over WikiLeaks' publication of secret US military documents and diplomatic cables in 2010.
US Attorney General Jeff Sessions last year said his arrest was a "priority".
But the judge rejected this, adding: "He appears to consider himself above the normal rules of law and wants justice only if it goes in his favour."
Gareth Peirce, one of Assange's lawyers, told AFP it would be possible to appeal against the judge's decision, but she said this had not yet been decided.
'Akin to imprisonment'
Assange wrote on Twitter before the hearing that his latest legal argument hinged on an alleged "cover-up" by the British government to keep him detained.
He highlighted newspaper report which quoted emails sent by Britain's Crown Prosecution Service (CPS) to Swedish counterparts in 2012, urging them not to drop their application for a European arrest warrant.
In the emails, a CPS lawyer apparently commented on a 2012 article saying that Sweden was dropping the case by writing: "Don't you dare get cold feet!"
Swedish prosecutors told the CPS in 2013 that they "felt obliged" to lift the warrant, but only announced last year that it had finally been dropped.
The CPS also admitted it destroyed emails relating to the case after the lawyer handling the British response retired in 2014.
Ecuador's foreign ministry said it wants to reach a solution with Britain that satisfies both sides and respects Assange's human rights.
Assange's lawyer Mark Summers said his client had been living in conditions "akin to imprisonment" and his "psychological health" has deteriorated and was "in serious peril".
The court heard that the 46-year-old was suffering from a bad tooth, a frozen shoulder and depression.
Assange only very rarely emerges onto the balcony of the embassy building, citing concerns for his personal safety, but he frequently takes part in media conferences and campaigns via video link.
Ecuador in December granted citizenship to the Australian-born Assange, and asked Britain to recognise him as a diplomat in an unsuccessful attempt to provide him with the immunity that could see him walk free.
But London swiftly rejected the move.
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